Prepare gif decoder for use in conjunction with gif demuxer.

      Add capability of reading multiple frames instead of only first.
Implement support for different gif frame 'disposal methods'.
Add option that allows to change background color resulting from
conversion of gif with transparency to any other format which
not support it.
Also bump lavc minor version.

Signed-off-by: Vitaliy E Sugrobov <vsugrob@hotmail.com>