Bug 759457. Don't fast path shadows with negative scale. r=roc

We don't support this yet, so don't bother trying.

layout/base/nsCSSRendering.cpp

@@ -4141,12 +4141,12 @@ nsContextBoxBlur::Init(const nsRect& aRect, nscoord aSpreadRadius,
   gfxFloat scaleX = 1;
   gfxFloat scaleY = 1;
 
-  // Do blurs in device space when possible
-  // If the scale is not uniform we fall back to transforming on paint.
+  // Do blurs in device space when possible.
   // Chrome/Skia always does the blurs in device space
   // and will sometimes get incorrect results (e.g. rotated blurs)
   gfxMatrix transform = aDestinationCtx->CurrentMatrix();
-  if (transform.HasNonAxisAlignedTransform()) {
+  // XXX: we could probably handle negative scales but for now it's easier just to fallback
+  if (transform.HasNonAxisAlignedTransform() || transform.xx <= 0.0 || transform.yy <= 0.0) {
     transform = gfxMatrix();
   } else {
     scaleX = transform.xx;