Bug 1359842 - Do some cleanup of GetWrRelBounds. r=nical

This moves the bulk of the meaningful work done by GetWrRelBounds into
strongly-typed helper functions. GetWrRelBounds is left as a wrapper so
call sites don't need to be updated yet. Note also that the
strongly-typed functions do not do any conversions from one reference
point from another (e.g. by calling the RelativeToXXX functions).

MozReview-Commit-ID: B3nPbOJOf9o

gfx/layers/wr/WebRenderLayer.cpp

@@ -11,6 +11,7 @@
 #include "mozilla/layers/WebRenderBridgeChild.h"
 #include "mozilla/layers/WebRenderLayerManager.h"
 #include "nsThreadUtils.h"
+#include "UnitTransforms.h"
 
 namespace mozilla {
 
@@ -144,27 +145,39 @@ WebRenderLayer::GetWrClipRect(gfx::Rect& aRect)
   return clip;
 }
 
-gfx::Matrix4x4
-WebRenderLayer::GetWrBoundTransform()
+LayerRect
+WebRenderLayer::Bounds()
+{
+  return LayerRect(GetLayer()->GetVisibleRegion().GetBounds());
+}
+
+BoundsTransformMatrix
+WebRenderLayer::BoundsTransform()
 {
   gfx::Matrix4x4 transform = GetLayer()->GetTransform();
   transform._41 = 0.0f;
   transform._42 = 0.0f;
   transform._43 = 0.0f;
-  return transform;
+  return ViewAs<BoundsTransformMatrix>(transform);
 }
 
-gfx::Rect
-WebRenderLayer::GetWrRelBounds()
+LayerRect
+WebRenderLayer::BoundsForStackingContext()
 {
-  gfx::Rect bounds = IntRectToRect(GetLayer()->GetVisibleRegion().GetBounds().ToUnknownRect());
-  gfx::Matrix4x4 transform = GetWrBoundTransform();
+  LayerRect bounds = Bounds();
+  BoundsTransformMatrix transform = BoundsTransform();
   if (!transform.IsIdentity()) {
     // WR will only apply the 'translate' of the transform, so we need to do the scale/rotation manually.
     bounds.MoveTo(transform.TransformPoint(bounds.TopLeft()));
   }
 
-  return RelativeToParent(bounds);
+  return bounds;
+}
+
+gfx::Rect
+WebRenderLayer::GetWrRelBounds()
+{
+  return RelativeToParent(BoundsForStackingContext().ToUnknownRect());
 }
 
 Maybe<wr::ImageKey>

gfx/layers/wr/WebRenderLayer.h

@@ -17,6 +17,8 @@ class ImageClientSingle;
 class WebRenderBridgeChild;
 class WebRenderLayerManager;
 
+typedef gfx::Matrix4x4Typed<LayerPixel, LayerPixel> BoundsTransformMatrix;
+
 class WebRenderLayer
 {
 public:
@@ -52,10 +54,13 @@ public:
   gfx::Point GetOffsetToParent();
   gfx::Rect TransformedVisibleBoundsRelativeToParent();
 protected:
+  LayerRect Bounds();
+  BoundsTransformMatrix BoundsTransform();
+  LayerRect BoundsForStackingContext();
+
   gfx::Rect GetWrBoundsRect();
   gfx::Rect GetWrRelBounds();
   gfx::Rect GetWrClipRect(gfx::Rect& aRect);
-  gfx::Matrix4x4 GetWrBoundTransform();
   void DumpLayerInfo(const char* aLayerType, gfx::Rect& aRect);
   Maybe<WrImageMask> BuildWrMaskLayer(bool aUnapplyLayerTransform);
 };