Fix AVI codecs in MKV which require a bit depth value to be set.
Fix a crash with DVD subtitles on x64 builds with software decoding.

Some code was forgotten in 0c8c08104c3e0d92529a01701260254704740241.
Convert remaining parts of the subtitle blending to ptrdiff_t
Fix palette for VobSub tracks in MP4.

FFmpeg now outputs the palette converted to RGB instead of the raw MP4-specific YUV palette.