ffmpeg: accept "chapters" as forced key frames.

Allow to force a key frame at the beginning of each chapter.
sws: Fix error diffusion flag

The used flag was mistakenly overlapping with chroma drop

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
sws: Check for unsupported combination of ED/FULL_CHR flags

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
sws: check that dither array is not null before memset

Fixes null pointer dereference

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