h264parse: mark SEI Recovery Point as keyframes

h264parse: mark SEI Recovery Point as keyframes

The spec states that "recovery point SEI message assists a decoder in
determining when the decoding process will produce acceptable
pictures for display after the decoder initiates random access or after the
encoder indicates a broken link in the coded video sequence."

Mark those as keyframes so muxers will mark them as seek points and
decoders will be able to start decoding from them rather than waiting
for an IDR.

https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/issues/790