vaapipostproc not allowing overriding colorimetry (bt601/709) by downstream caps

Submitted by Florent Thiery @florent.thiery

Link to original bug (#791876)

Description

I am decoding with vaapidecodebin (autoplugged through decodebin), which scales automagically. However i need bt709 support to avoid an additional conversion between bt601 and bt709, or the following will crash:

$ gst-launch-1.0 videotestsrc num-buffers=1 ! x264enc ! h264parse ! vaapidecodebin ! "video/x-raw, format=(string)NV12, width=(int)960, height=(int)540, framerate=(fraction)30, colorimetry=(string)bt709, pixel-aspect-ratio=1/1" ! fakesink

Works:

$ gst-launch-1.0 videotestsrc num-buffers=1 ! x264enc ! h264parse ! vaapidecodebin ! videoconvert ! "video/x-raw, format=(string)NV12, width=(int)960, height=(int)540, framerate=(fraction)30, colorimetry=(string)bt709, pixel-aspect-ratio=1/1" ! fakesink

Is it possible to support bt709 ?