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

test/video_source.h

@@ -50,6 +50,15 @@ static FILE *OpenTestDataFile(const std::string& file_name) {
   return fopen(path_to_source.c_str(), "rb");
 }
 
+static FILE *OpenTestOutFile(const std::string& file_name) {
+  const std::string path_to_source = GetDataPath() + "/" + file_name;
+  return fopen(path_to_source.c_str(), "wb");
+}
+
+static FILE *OpenTempOutFile() {
+  return tmpfile();
+}
+
 // Abstract base class for test video sources, which provide a stream of
 // vpx_image_t images with associated timestamps and duration.
 class VideoSource {