Allow safe downcasting to ContentTextureHost and ImageHost. (bug 1365879 part 5, r=mattwoodrow)

gfx/layers/composite/CompositableHost.h

@@ -39,11 +39,13 @@ namespace layers {
 
 class Layer;
 class LayerComposite;
+class ImageHost;
 class Compositor;
 class ThebesBufferData;
 class TiledContentHost;
 class CompositableParentManager;
 class WebRenderImageHost;
+class ContentHostTexture;
 struct EffectChain;
 
 struct ImageCompositeNotificationInfo {
@@ -146,6 +148,8 @@ public:
   Layer* GetLayer() const { return mLayer; }
   void SetLayer(Layer* aLayer) { mLayer = aLayer; }
 
+  virtual ContentHostTexture* AsContentHostTexture() { return nullptr; }
+  virtual ImageHost* AsImageHost() { return nullptr; }
   virtual TiledContentHost* AsTiledContentHost() { return nullptr; }
   virtual WebRenderImageHost* AsWebRenderImageHost() { return nullptr; }
 

gfx/layers/composite/ContentHost.h

@@ -177,6 +177,8 @@ public:
     mLocked = false;
   }
 
+  ContentHostTexture* AsContentHostTexture() override { return this; }
+
   virtual already_AddRefed<TexturedEffect> GenEffect(const gfx::SamplingFilter aSamplingFilter) override;
 
 protected:

gfx/layers/composite/ImageHost.h

@@ -44,6 +44,7 @@ public:
   ~ImageHost();
 
   virtual CompositableType GetType() override { return mTextureInfo.mCompositableType; }
+  virtual ImageHost* AsImageHost() override { return this; }
 
   virtual void Composite(Compositor* aCompositor,
                          LayerComposite* aLayer,