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.
[MPC-HC] Use our own ffmpeg clone repository.

WARNING: This should be the first custom commit in our fork and its title should not be modified.
[MPC-HC] LAV Splitter: Exhibits MKV attachments through IDSMResourceBag interface.
[MPC-HC] Tray icon: Add an optional custom callback for "show property page" events.
[MPC-HC] Fix the include directories for FFmpeg's out-of-source-tree build.
[MPC-HC] Version number: Add a "git describe based" commit number to identify the vanilla LAV Filters we are based on.

This way if we distribute LAV Filters version X.Y.Z.N, it means that our custom build is based on:
 - the stable version X.Y.Z,
 - with N official patches (which will be part of next official LAV Filters build),
 - and some custom non-official patches which are here to improve the integration in MPC-HC.