jpegenc: do not proxy our filter caps downstream on caps queries
jpegenc: do not proxy our filter caps downstream on caps queries

Downstream likely won't accept video/x-raw and the caps query
will return EMPTY caps. Instead, create a copy of the caps that
has all structure names replaced by 'image/jpeg'

Simple pipeline that shows the problem:
gst-launch-1.0 videotestsrc num-buffers=1 ! "video/x-raw, \
  width=(int)640, height=(int)480" ! videoscale ! jpegenc ! \
  "image/jpeg, width=(int)800, height=(int)600" ! filesink \
  location=/tmp/image.jpg

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