Bug 1456555 - Fix SourceSurface offset handling in DTSkia::PushLayer. r=lsalzman

This got added in bug 1429508 and then removed in bug 1451845. Tiled blobs adds tests for this, so it shouldn't break again.

MozReview-Commit-ID: 3azL7SoWlr2

Depends on D10038

Differential Revision: https://phabricator.services.mozilla.com/D10041

--HG--
extra : moz-landing-system : lando

gfx/2d/DrawTargetSkia.cpp

@@ -2125,6 +2125,10 @@ DrawTargetSkia::PushLayerWithBlend(bool aOpaque, Float aOpacity, SourceSurface*
   sk_sp<SkImage> clipImage = aMask ? GetSkImageForSurface(aMask) : nullptr;
   SkMatrix clipMatrix;
   GfxMatrixToSkiaMatrix(aMaskTransform, clipMatrix);
+  if (aMask) {
+    clipMatrix.postTranslate(aMask->GetRect().X(), aMask->GetRect().Y());
+  }
+
   SkCanvas::SaveLayerRec saveRec(aBounds.IsEmpty() ? nullptr : &bounds,
                                  &paint,
                                  nullptr,