gifdec: resync support

This allows decoding gifs that have junk at the begin
and also allows byte wise seeking

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavfi/showwaves: add display mode option and mode=line
doc/filters: apply minor fixes
lavfi/showwaves: fail in case of push_frame() error
ffv1enc: fix gbrp>8bit

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>