Use double precision when computing the backdrop copy rect. (bug 1243071 part 2, r=mattwoodrow)

gfx/2d/Rect.h

@@ -184,8 +184,8 @@ struct RectTyped :
     {
       *aOut = IntRectTyped<units>(int32_t(this->X()), int32_t(this->Y()),
                                   int32_t(this->Width()), int32_t(this->Height()));
-      return RectTyped<units>(F(aOut->x), F(aOut->y),
-                              F(aOut->width), F(aOut->height))
+      return RectTyped<units, F>(F(aOut->x), F(aOut->y),
+                                 F(aOut->width), F(aOut->height))
              .IsEqualEdges(*this);
     }
 

gfx/layers/Compositor.cpp

@@ -369,11 +369,13 @@ Compositor::ComputeBackdropCopyRect(const gfx::Rect& aRect,
   renderBounds.MoveBy(rtOffset);
 
   // Apply the layer transform.
-  gfx::Rect dest = aTransform.TransformAndClipBounds(aRect, renderBounds);
+  gfx::RectDouble dest = aTransform.TransformAndClipBounds(
+    gfx::RectDouble(aRect.x, aRect.y, aRect.width, aRect.height),
+    gfx::RectDouble(renderBounds.x, renderBounds.y, renderBounds.width, renderBounds.height));
   dest -= rtOffset;
 
   // Ensure we don't round out to -1, which trips up Direct3D.
-  dest.IntersectRect(dest, gfx::Rect(0, 0, rtSize.width, rtSize.height));
+  dest.IntersectRect(dest, gfx::RectDouble(0, 0, rtSize.width, rtSize.height));
 
   // Round out to integer.
   gfx::IntRect result;