lavf/gif: simplify streams type checking.
lavf/gif: remove unused fields.
lavf/gif: trim unnecessarily long netscape ext code.

Extension description comments are now placed along the avio calls, the
always defined macro removed, and the always true loop_count check as
well (loop option is bound to 0-65535).
lavf/gif: merge gif_write_{packet,video}.

Avoid a pointless split.
lavf/gif: support only GIF codec.
lavc/gif: avoid encoding 0x0 images.

It seems browsers don't like it very much.
evrcdec: use memmove() instead of memcpy() when regions can overlap.

This occurs also with valid files.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>