h264: we should check first_mb_in_slice when decoding

[dwlsalmeida]

h264: we should check first_mb_in_slice when decoding

[dwlsalmeida]

It would be great if we could assert! or debug_assert! these two conditions:

–If separate_colour_plane_flag is equal to 0, the value of first_mb_in_slice shall not be less than the value of
first_mb_in_slice for any other slice of the current picture that precedes the current slice in decoding order.

–Otherwise (separate_colour_plane_flag is equal to 1), the value of first_mb_in_slice shall not be less than the value
of first_mb_in_slice for any other slice of the current picture that precedes the current slice in decoding order and has
the same value of colour_plane_id.

[Gnurou]

Reopening to address the comments right above.