Bug 1370117 - Update the external image status for reusing case. r=sotaro

When gecko go through the reusing case, the aUseExternalImage isn't set to the correct value. It is always false here.
Then, gecko will use the non-external-image code path later. Add a new data member "mUseExternalImage" to save the previous status.

MozReview-Commit-ID: KvzgIzYXNAB

gfx/layers/wr/WebRenderCompositableHolder.cpp

@@ -20,6 +20,7 @@ namespace layers {
 WebRenderCompositableHolder::AsyncImagePipelineHolder::AsyncImagePipelineHolder()
  : mInitialised(false)
  , mIsChanged(false)
+ , mUseExternalImage(false)
  , mFilter(WrImageRendering::Auto)
  , mMixBlendMode(WrMixBlendMode::Normal)
 {}
@@ -293,6 +294,7 @@ WebRenderCompositableHolder::UpdateImageKeys(wr::WebRenderAPI* aApi,
   if (texture == aHolder->mCurrentTexture) {
     // Reuse previous ImageKeys.
     aKeys.AppendElements(aHolder->mKeys);
+    aUseExternalImage = aHolder->mUseExternalImage;
     return true;
   }
 
@@ -306,7 +308,7 @@ WebRenderCompositableHolder::UpdateImageKeys(wr::WebRenderAPI* aApi,
     return true;
   }
 
-  aUseExternalImage = GetImageKeyForTextureHost(aApi, texture, aKeys);
+  aUseExternalImage = aHolder->mUseExternalImage = GetImageKeyForTextureHost(aApi, texture, aKeys);
   MOZ_ASSERT(!aKeys.IsEmpty());
   aHolder->mKeys.AppendElements(aKeys);
   aHolder->mCurrentTexture = texture;

gfx/layers/wr/WebRenderCompositableHolder.h

@@ -122,6 +122,7 @@ private:
 
     bool mInitialised;
     bool mIsChanged;
+    bool mUseExternalImage;
     LayerRect mScBounds;
     gfx::Matrix4x4 mScTransform;
     gfx::MaybeIntSize mScaleToSize;