h264/5parse: mark SEI Recovery Point as keyframes

Submitted by Guillaume Desmottes @gdesmott

Link to original bug (#797216)

Description

The AVC and HEVC 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.