libvpx

Adds support for reading and writing 10/12-bit y4m

The y4m extension used is the same as the one used in ffmpeg/x264.
The patch is adapted from the highbitdepth branch.

Also adds unit tests for y4m header parsing and md5 check
of the raw frame data, as well as y4m writing.

Change-Id: Ie2794daf6dbafd2f128464f9b9da520fc54c0dd6