Bug 1450307 - Round scaled paint size instead of paint bounds in GenerateFallbackData. r=jrmuizel

A component of dtSize could be zero because RoundedToInt(rect).Size() is not the same as
RoundedToInt(rect.Size()). The size after rounding might be zero, even if the rounded size
is not zero. For example with x0=0.5, x1=1.1.

MozReview-Commit-ID: 3NeBMAVD5ub

--HG--
extra : rebase_source : a241b10aa6e1e788691cfb7ac412fe5073257c81
extra : amend_source : 8d55d933c021b77463eb6c4fe0a2503fc9bf78d7

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -1609,9 +1609,9 @@ WebRenderCommandBuilder::GenerateFallbackData(nsDisplayItem* aItem,
 
   // Some display item may draw exceed the paintSize, we need prepare a larger
   // draw target to contain the result.
-  auto scaledBounds = bounds * LayoutDeviceToLayerScale(1);
-  scaledBounds.Scale(scale.width, scale.height);
-  LayerIntSize dtSize = RoundedToInt(scaledBounds).Size();
+  auto scaledPaintSize = bounds.Size() * LayoutDeviceToLayerScale(1);
+  scaledPaintSize.Scale(scale.width, scale.height);
+  LayerIntSize dtSize = RoundedToInt(scaledPaintSize);
 
   bool needPaint = true;
   LayoutDeviceIntPoint offset = RoundedToInt(bounds.TopLeft());