[FFmpeg-devel] Pixel format support fixes in swscale and drawutils

[rcombs]

This patchset is also available as a GitHub pull request for review simplicity:
https://github.com/FFmpeg/FFmpeg/pull/380

- Reduce hardcoding of pixfmt lists, preferring deriving properties from pixdesc
- Fix big-endian support for P[N]10/P[N]16 in swscale
- Fix several drawutils issues and add new pixfmt support
- Add more defensive checks in drawutils