Bug 1137529: Prefer Apple's VDA hardware acceleration for Hi-Def videos. r=rillian

With some GPUs (such as Intel HD-x000), Apple VideoTool box provides poor
decoding speed, causing us to drop frames for most HD videos.
VDA is around 50 times faster on those machines (31ms average to decode a 4K
frame with VT, while 0.6ms average with VDA)

dom/media/fmp4/apple/AppleDecoderModule.cpp

@@ -27,6 +27,9 @@ PRLogModuleInfo* GetAppleMediaLog() {
 
 namespace mozilla {
 
+// This defines the resolution height over which VDA will be prefered.
+#define VDA_RESOLUTION_THRESHOLD 720
+
 bool AppleDecoderModule::sInitialized = false;
 bool AppleDecoderModule::sIsVTAvailable = false;
 bool AppleDecoderModule::sIsVTHWAvailable = false;
@@ -47,12 +50,12 @@ AppleDecoderModule::Init()
 {
   MOZ_ASSERT(NS_IsMainThread(), "Must be on main thread.");
 
-  sForceVDA = Preferences::GetBool("media.apple.forcevda", false);
-
   if (sInitialized) {
     return;
   }
 
+  Preferences::AddBoolVarCache(&sForceVDA, "media.apple.forcevda", false);
+
   // dlopen VideoDecodeAcceleration.framework if it's available.
   sIsVDAAvailable = AppleVDALinker::Link();
 
@@ -163,7 +166,9 @@ AppleDecoderModule::CreateVideoDecoder(const mp4_demuxer::VideoDecoderConfig& aC
 {
   nsRefPtr<MediaDataDecoder> decoder;
 
-  if (sIsVDAAvailable && (!sIsVTHWAvailable || sForceVDA)) {
+  if (sIsVDAAvailable &&
+      (!sIsVTHWAvailable || sForceVDA ||
+       aConfig.image_height >= VDA_RESOLUTION_THRESHOLD)) {
     decoder =
       AppleVDADecoder::CreateVDADecoder(aConfig,
                                         aVideoTaskQueue,