h264: make ff_h264_frame_start static.

It is not called from outside h264.c
h264: merge common_init() into ff_h264_decode_init.

There is no point in keeping those separate.
h264: add a parameter to the MB_MBAFF macro.

This way it does not look like a constant.
h264: add a parameter to the MB_FIELD macro.

This way it does not look like a constant.
h264: add a parameter to the FRAME_MBAFF macro.

This way it does not look like a constant.
h264: add a parameter to the FIELD_PICTURE macro.

This way it does not look like a constant.
h264: add a parameter to the FIELD_OR_MBAFF_PICTURE macro.

This way it does not look like a constant.
h264: add a parameter to the CABAC macro.

This way it does not look like a constant.
h264: add a parameter to the CHROMA422 macro.

This way it does not look like a constant.
h264: add a parameter to the CHROMA444 macro.

This way it does not look like a constant.
h264: remove redundant freeing of DPB in h264_decode_end

free_tables() frees it already.