v4l2videodec: Keep part of the input buffer
v4l2videodec: Keep part of the input buffer

Instead of completely getting rid of the input buffer, copy
the metadata, the flags and the timestamp into an empty buffer.
This way the decoder base class can copy that information again
to the output buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=758424

StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good