avcodec: sync mt decoders to the process thread

This ensures smoother operation in real-world situations, especially
transcoding or AviSynth processing.
dxva2: ensure we sync to the process thread if required
set custom io flag to prevent a crash when releasing a fresh context
Block opening .ifo files
Add more DV FourCCs
Update ffmpeg for small mkv seeking improvement
Update ffmpeg for further mkv seek fixes
And another small mkv tweak
Prefer string type macros in external interfaces
Mark input-only strings as constant in the public interface
Add RGB48 as an internal pixel format
Update ffmpeg
Use AV_PIX_FMT_* instead of the deprecated PIX_FMT_*
Add mappings for new pixel formats
Use AVPixelFormat instead of deprecated PixelFormat and disable old names
Rename macros to be more generic
Add SSSE3 RGB48->RGB32/RGB24 converter
Restructure SSSE3 RGB48 converter for better readability
The RGB48->RGB24 converter doesn't actually use SSSE3
Update ffmpeg for small build fix