lsmashsource: Add 'format' option to force output pixel format.

Currently, the following formats are supported.
  YUV420P8
  YUV422P8
  YUV444P8
  YUV410P8
  YUV411P8
  YUV420P9
  YUV422P9
  YUV444P9
  YUV420P10
  YUV422P10
  YUV444P10
  YUV420P16
  YUV422P16
  YUV444P16