v4l2sink: special handling for cases gst_buffer_make_metadata_writable()
v4l2sink: special handling for cases gst_buffer_make_metadata_writable()

Special case check for sub-buffers:  In certain cases, places like
GstBaseTransform, which might check that the buffer is writable before copying
metadata, timestamp, and such, will find that the buffer has more than one
reference to it.  In these cases, they will create a sub-buffer with an offset=0
and length equal to the original buffer size.

This could happen in two scenarios: (1) a tee in the pipeline, and (2) because
the refcnt is incremented in gst_mini_object_free() before the finalize function
is called, and decremented after it returns..  but returning this buffer to the
buffer pool in the finalize function, could wake up a thread blocked in
_buffer_alloc() which could run and get a buffer w/ refcnt==2 before the thread
originally unref'ing the buffer returns from finalize function and decrements
the refcnt back to 1!

This is related to issue #545501

ted-n/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,strukturag/gst-plugins-good,alessandrod/gst-plugins-good,GStreamer/gst-plugins-good,Lachann/gst-plugins-good,luisbg/gst-plugins-good,sebras/gst-plugins-good,ndufresne/gst-plugins-good,knuesel/gst-plugins-good,kittee/gst-plugins-good,froggatt/gst-plugins-good-m,loshca/gst-plugins-good,krieger-od/gst-plugins-good,greg80303/gst-plugins-good,cfoch/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,BigBrother-International/gst-plugins-good,cablelabs/gst-plugins-good,veo-labs/gst-plugins-good,knuesel/gst-plugins-good,pexip/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,davibe/gst-plugins-good-1.0,mrchapp/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,wkatsak/gst-plugins-good,loshca/gst-plugins-good,matsu/gst-plugins-good,Lachann/gst-plugins-good,ariscop/gst-plugins-good,jcaden/gst-plugins-good,StreamUtils/gst-plugins-good,offlinehacker/gst-plugins-good,Kurento/gst-plugins-good,strukturag/gst-plugins-good,dgerlach/gst-plugins-good,pexip/gst-plugins-good,jcaden/gst-plugins-good,krieger-od/gst-plugins-good,an146/gst-plugins-good,BigBrother-International/gst-plugins-good,veo-labs/gst-plugins-good,reynaldo-samsung/gst-plugins-good,jhodapp/gst-plugins-good,GrokImageCompression/gst-plugins-good,rawoul/gst-plugins-good,stfl/gst-plugins-good,davibe/gst-plugins-good,alessandrod/gst-plugins-good,krieger-od/gst-plugins-good,ikonst/gst-plugins-good,offlinehacker/gst-plugins-good,shelsonjava/gst-plugins-good,stfl/gst-plugins-good,sh0/gst-plugins-good,krad-radio/gstreamer-plugins-good-krad,davibe/gst-plugins-good,jhodapp/gst-plugins-good,veo-labs/gst-plugins-good,lovebug356/gst-plugins-good,cablelabs/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,krad-radio/gstreamer-plugins-good-krad,hizukiayaka/gst-plugins-good,alessandrod/gst-plugins-good,loshca/gst-plugins-good,kittee/gst-plugins-good,dgerlach/gst-plugins-good,strukturag/gst-plugins-good,Distrotech/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,cablelabs/gst-plugins-good,reynaldo-samsung/gst-plugins-good,surround-io/gst-plugins-good,an146/gst-plugins-good,veo-labs/gst-plugins-good,ikonst/gst-plugins-good,mrchapp/gst-plugins-good,wkatsak/gst-plugins-good,rikaunite/gst-opera_gst-plugins-good,cablelabs/gst-plugins-good,ahmedammar/platform_external_gst_plugins_good,strukturag/gst-plugins-good,cfoch/gst-plugins-good,knuesel/gst-plugins-good,ahmedammar/platform_external_gst_plugins_good,krad-radio/gstreamer-plugins-good-krad,ariscop/gst-plugins-good,alessandrod/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,ahmedammar/platform_external_gst_plugins_good,stfl/gst-plugins-good,shelsonjava/gst-plugins-good,matsu/gst-plugins-good,jpakkane/gstreamer-plugins-good,collects/gst-plugins-good,cfoch/gst-plugins-good,lovebug356/gst-plugins-good,ylatuya/gst-plugins-good,cfoch/gst-plugins-good,collects/gst-plugins-good,reynaldo-samsung/gst-plugins-good,davibe/gst-plugins-good-1.0,surround-io/gst-plugins-good,surround-io/gst-plugins-good,jpakkane/gstreamer-plugins-good,mrchapp/gst-plugins-good,Distrotech/gst-plugins-good,jcaden/gst-plugins-good,krieger-od/gst-plugins-good,GStreamer/gst-plugins-good,ylatuya/gst-plugins-good,GrokImageCompression/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,ahmedammar/platform_external_gst_plugins_good,davibe/gst-plugins-good-1.0,lovebug356/gst-plugins-good,matsu/gst-plugins-good,GStreamer/gst-plugins-good,jhodapp/gst-plugins-good,krad-radio/gstreamer-plugins-good-krad,sebras/gst-plugins-good,hizukiayaka/gst-plugins-good,loshca/gst-plugins-good,rikaunite/gst-opera_gst-plugins-good,vatavuserban/gst-plugins-good,vatavuserban/gst-plugins-good,pexip/gst-plugins-good,rawoul/gst-plugins-good,ted-n/gst-plugins-good,collects/gst-plugins-good,matsu/gst-plugins-good,GStreamer/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,chamois94/gst-plugins-good,jhodapp/gst-plugins-good,sebras/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,surround-io/gst-plugins-good,Distrotech/gst-plugins-good,vatavuserban/gst-plugins-good,ndufresne/gst-plugins-good,kittee/gst-plugins-good,an146/gst-plugins-good,chamois94/gst-plugins-good,jpakkane/gstreamer-plugins-good,ndufresne/gst-plugins-good,sh0/gst-plugins-good,wkatsak/gst-plugins-good,BigBrother-International/gst-plugins-good,ted-n/gst-plugins-good,luisbg/gst-plugins-good,ikonst/gst-plugins-good,ndufresne/gst-plugins-good,knuesel/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,lovebug356/gst-plugins-good,offlinehacker/gst-plugins-good,shelsonjava/gst-plugins-good,greg80303/gst-plugins-good,wkatsak/gst-plugins-good,Lachann/gst-plugins-good,mrchapp/gst-plugins-good,davibe/gst-plugins-good-1.0,collects/gst-plugins-good,hizukiayaka/gst-plugins-good,froggatt/gst-plugins-good-m,BigBrother-International/gst-plugins-good,vatavuserban/gst-plugins-good,ylatuya/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,mrchapp/gst-plugins-good,ariscop/gst-plugins-good,Distrotech/gst-plugins-good,hizukiayaka/gst-plugins-good,rawoul/gst-plugins-good,chamois94/gst-plugins-good,offlinehacker/gst-plugins-good,StreamUtils/gst-plugins-good,kittee/gst-plugins-good,luisbg/gst-plugins-good,sh0/gst-plugins-good,froggatt/gst-plugins-good-m,GrokImageCompression/gst-plugins-good,dgerlach/gst-plugins-good,jpakkane/gstreamer-plugins-good,froggatt/gst-plugins-good-m,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,ted-n/gst-plugins-good,luisbg/gst-plugins-good,greg80303/gst-plugins-good,ylatuya/gst-plugins-good,chamois94/gst-plugins-good,rawoul/gst-plugins-good,Lachann/gst-plugins-good,jcaden/gst-plugins-good,reynaldo-samsung/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,greg80303/gst-plugins-good,sh0/gst-plugins-good,shelsonjava/gst-plugins-good,rikaunite/gst-opera_gst-plugins-good,sebras/gst-plugins-good,rikaunite/gst-opera_gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,ariscop/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,an146/gst-plugins-good,davibe/gst-plugins-good,Distrotech/gst-plugins-good,pexip/gst-plugins-good