Fix PrepareDrawTargetForPainting (bug 1409871 part 16, r=dvander)

CapturedPaintState::mTarget is actually a dual draw target to the black and
white draw targets, so this code will have the white draw target cleared black
and then white. This isn't incorrect, it's just wasteful.

MozReview-Commit-ID: ItgiSmegPK6

--HG--
extra : rebase_source : 9c3c88f463087a75383f1940ddeef6bb68ec35c4

gfx/layers/PaintThread.cpp

@@ -30,7 +30,7 @@ struct MOZ_STACK_CLASS AutoCapturedPaintSetup
 {
   AutoCapturedPaintSetup(CapturedPaintState* aState, CompositorBridgeChild* aBridge)
   : mState(aState)
-  , mTarget(aState->mTarget)
+  , mTarget(aState->mTargetDual)
   , mRestorePermitsSubpixelAA(mTarget->GetPermitSubpixelAA())
   , mOldTransform(mTarget->GetTransform())
   , mBridge(aBridge)
@@ -186,7 +186,7 @@ PaintThread::AsyncPaintContents(CompositorBridgeChild* aBridge,
   MOZ_ASSERT(IsOnPaintThread());
   MOZ_ASSERT(aState);
 
-  DrawTarget* target = aState->mTarget;
+  DrawTarget* target = aState->mTargetDual;
   DrawTargetCapture* capture = aState->mCapture;
 
   AutoCapturedPaintSetup setup(aState, aBridge);

gfx/layers/PaintThread.h

@@ -28,12 +28,14 @@ class CapturedPaintState {
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(CapturedPaintState)
 public:
   CapturedPaintState(nsIntRegion& aRegionToDraw,
+                     gfx::DrawTarget* aTargetDual,
                      gfx::DrawTarget* aTarget,
                      gfx::DrawTarget* aTargetOnWhite,
                      const gfx::Matrix& aTargetTransform,
                      SurfaceMode aSurfaceMode,
                      gfxContentType aContentType)
   : mRegionToDraw(aRegionToDraw)
+  , mTargetDual(aTargetDual)
   , mTarget(aTarget)
   , mTargetOnWhite(aTargetOnWhite)
   , mTargetTransform(aTargetTransform)
@@ -45,6 +47,7 @@ public:
   RefPtr<TextureClient> mTextureClient;
   RefPtr<TextureClient> mTextureClientOnWhite;
   RefPtr<gfx::DrawTargetCapture> mCapture;
+  RefPtr<gfx::DrawTarget> mTargetDual;
   RefPtr<gfx::DrawTarget> mTarget;
   RefPtr<gfx::DrawTarget> mTargetOnWhite;
   gfx::Matrix mTargetTransform;

gfx/layers/client/ClientPaintedLayer.cpp

@@ -224,7 +224,7 @@ ClientPaintedLayer::PaintOffMainThread()
   while (RefPtr<CapturedPaintState> captureState =
           mContentClient->BorrowDrawTargetForRecording(state, &iter))
   {
-    DrawTarget* target = captureState->mTarget;
+    DrawTarget* target = captureState->mTargetDual;
     if (!target || !target->IsValid()) {
       if (target) {
         mContentClient->ReturnDrawTargetToBuffer(target);

gfx/layers/client/ContentClient.cpp

@@ -253,7 +253,7 @@ ContentClient::BorrowDrawTargetForPainting(ContentClient::PaintState& aPaintStat
     return nullptr;
   }
 
-  return capturedState->mTarget;
+  return capturedState->mTargetDual;
 }
 
 RefPtr<CapturedPaintState>
@@ -282,6 +282,7 @@ ContentClient::BorrowDrawTargetForRecording(ContentClient::PaintState& aPaintSta
   RefPtr<CapturedPaintState> state =
     new CapturedPaintState(regionToDraw,
                            result,
+                           mBuffer->GetDTBuffer(),
                            mBuffer->GetDTBufferOnWhite(),
                            transform,
                            aPaintState.mMode,