Bug 1477260 - Clip the rect in Matrix4x4TypedFlagged::TransformAndClipBounds, when transform is an identity matrix r=bas

MozReview-Commit-ID: 3a8vnl3K87n

--HG--
extra : rebase_source : b4bdd2b14e10becee20007e453497e9d7a204097

gfx/2d/Matrix.h

@@ -1907,7 +1907,9 @@ public:
                                                    const RectTyped<TargetUnits, F>& aClip) const
   {
     if (mType == MatrixType::Identity) {
-      return aRect;
+      const RectTyped<SourceUnits, F>& clipped = aRect.Intersect(aClip);
+      return RectTyped<TargetUnits, F>(clipped.X(), clipped.Y(),
+                                       clipped.Width(), clipped.Height());
     }
 
     if (mType == MatrixType::Simple) {