Support for FFmpeg's extended Y4M format

[Former user]

FFmpeg has supported >8-bit pixel formats in their yuv4mpeg encoder and decoder since the middle of 2012 (commits f7f6aaf9 and f91f049a, most significantly), and x264 has recognized said extensions since November of 2012 (commit 28ddb0dd).

Opening an issue for this since it's been brought up a few times in x265's discussion thread on Doom9 but there's been no sort of official response concerning why this isn't supported yet, which just leads to speculation and conjecture, as well as forcing users wanting to input >8-bit into x265 itself (rather than the libx265 wrapper in libavcodec) to use raw YUV, which is more inconvenient than Y4M due to requiring extra CLI parameters.

[Steve Borho]

y4m: support variable bit depth via CXXXpDD Y4MPEG header; ie: C420p10

ffmpeg's support for this is non-standard, so you must use -strict -1, aka:

ffmpeg -i vid.avi -pix_fmt yuv420p10le -strict -1 -f yuv4mpegpipe - | ./x265 - --y4m o.hevc

closes issue #53

→ <<cset c4adcaef8d1d>>