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

y4menc.h

@@ -23,7 +23,7 @@ extern "C" {
 
 int y4m_write_file_header(char *buf, size_t len, int width, int height,
                           const struct VpxRational *framerate,
-                          vpx_img_fmt_t fmt);
+                          vpx_img_fmt_t fmt, unsigned int bit_depth);
 int y4m_write_frame_header(char *buf, size_t len);
 
 #ifdef __cplusplus