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.

[build fix for Mac/VS by not using tuples with strings]

Change-Id: I40897ee37d289e4b6cea6fedc67047d692b8cb46
diff --git a/tools_common.h b/tools_common.h
index e033de2..6a9f4f7 100644
--- a/tools_common.h
+++ b/tools_common.h
@@ -90,6 +90,7 @@
   uint32_t width;
   uint32_t height;
   vpx_img_fmt_t fmt;
+  vpx_bit_depth_t bit_depth;
   int only_i420;
   uint32_t fourcc;
   struct VpxRational framerate;