h264: decouple h264_ps from the h264 decoder

h264: decouple h264_ps from the h264 decoder

Make the SPS/PPS parsing independent of the H264Context, to allow
decoupling the parser from the decoder. The change is modelled after the
one done earlier for HEVC.

Move the dequant buffers to the PPS to avoid complex checks whether they
changed and an expensive copy for frame threads.