codec/h264/h264dec/decode.go: decoding process for macroblock to slice group map

[Saxon Milton]

Implement the decoding process for macroblock to slice group map specified in section 8.2.2 with subsections 8.2.2.1, 8.2.2.2, 8.2.2.3, 8.2.2.4, 8.2.2.5, 8.2.2.6, 8.2.2.7, 8.2.2.8 (relevant pages attached)

[Saxon Milton]

Efforts towards H.264 decoder have been put on hold.

[Trek Hopton]

PR #255 was for this issue but didn’t get merged.