Allows one to do:

    ffmpeg -s 1920x1080          -i blah.v210 ...
    ffmpeg -s 1920x1080 -f v210x -i blah.yuv10 ...

Fixes #1869.
Reviewed-by: James Almer <jamrial@gmail.com>
Reviewed-by: Kieran Kunhya <kierank@obe.tv>