mpeg4videodec: replace MpegEncContext.reduced_res_vop with a local variable
mpeg4videodec: move new_pred from MpegEncContext to Mpeg4DecContext
mpeg4videodec: remove a write-only variable from MpegEncContext
mpeg4videodec: move enhancement_type from MpegEncContext to Mpeg4DecContext
mpeg4videodec: move scalability from MpegEncContext to Mpeg4DecContext
mpeg4videodec: move sprite_brightness_change from MpegEncContext to Mpeg4DecContext
mpeg4videodec: move num_sprite_warping_points from MpegEncContext to Mpeg4DecContext
mpeg4videodec: move sprite_{shift,traj} from MpegEncContext to Mpeg4DecContext
go2meeting: disallow tile dimensions that are not multiple of 16

Original decoder seems to always use 176x128 tiles anyway and this helps
avoiding lots of issues with odd tile sizes in fuzzed files.

Signed-off-by: Anton Khirnov <anton@khirnov.net>