lavc/h264: create AVFrame side data from H.264 timecodes

lavc/h264: create AVFrame side data from H.264 timecodes

Create SMPTE ST 12-1 timecodes based on H.264 SEI picture timing
info.

For framerates > 30 FPS, the field flag is used in conjunction with
pairs of frames which contain the same frame timestamp in S12M.
Ensure the field is properly set per the spec.