Bug 994548 - Fix masks being affected by partial updates in BasicCompositor. r=mattwoodrow

gfx/layers/basic/BasicCompositor.cpp

@@ -310,6 +310,7 @@ BasicCompositor::DrawQuad(const gfx::Rect& aRect,
     MOZ_ASSERT(effectMask->mMaskTransform.Is2D(), "How did we end up with a 3D transform here?!");
     MOZ_ASSERT(!effectMask->mIs3D);
     maskTransform = effectMask->mMaskTransform.As2D();
+    maskTransform.Translate(-offset.x, -offset.y);
   }
 
   switch (aEffectChain.mPrimaryEffect->mType) {