va: vapostproc does not implement caps negotiation correctly
# works
$ gst-launch-1.0 gltestsrc ! glcolorconvert ! gldownload ! video/x-raw,format=NV12 ! vapostproc ! vah264enc ! fakesink
# doesn't work because vapostproc can't convert RGBA->NV12 it seems
$ gst-launch-1.0 gltestsrc ! glcolorconvert ! gldownload ! vapostproc ! vah264enc ! fakesink