Issue #2357 - VPXDecoder does not decode alpha frames.

Another requirement to have transparency in videos. Straight port of Firefox 53 implementation, save for some oddities relating to needing to put uint8_t instead of uint8 in yuv_convert to get this to compile.

Ref: BZ 1321076, 1329104

dom/media/platforms/agnostic/VPXDecoder.h

@@ -57,6 +57,8 @@ private:
   void ProcessDecode(MediaRawData* aSample);
   MediaResult DoDecode(MediaRawData* aSample);
   void ProcessDrain();
+  MediaResult DecodeAlpha(vpx_image_t** aImgAlpha,
+                          MediaRawData* aSample);
 
   const RefPtr<ImageContainer> mImageContainer;
   const RefPtr<TaskQueue> mTaskQueue;
@@ -66,6 +68,9 @@ private:
   // VPx decoder state
   vpx_codec_ctx_t mVPX;
 
+  // VPx alpha decoder state
+  vpx_codec_ctx_t mVPXAlpha;
+
   const VideoInfo& mInfo;
 
   const Codec mCodec;