Bug 1368776 - Part 9. Expose getting an image container at a given size. r=tnikkel

This adds IsImageContainerAvailableAtSize and GetImageContainerAtSize to
the imgIContainer interface, as well as stubbing it for all of the
classes which implement it. The real implementations will follow for the
more complicated classes (RasterImage, VectorImage).

image/DynamicImage.cpp

@@ -226,6 +226,22 @@ DynamicImage::GetImageContainer(LayerManager* aManager, uint32_t aFlags)
   return nullptr;
 }
 
+NS_IMETHODIMP_(bool)
+DynamicImage::IsImageContainerAvailableAtSize(LayerManager* aManager,
+                                              const IntSize& aSize,
+                                              uint32_t aFlags)
+{
+  return false;
+}
+
+NS_IMETHODIMP_(already_AddRefed<ImageContainer>)
+DynamicImage::GetImageContainerAtSize(LayerManager* aManager,
+                                      const IntSize& aSize,
+                                      uint32_t aFlags)
+{
+  return nullptr;
+}
+
 NS_IMETHODIMP_(DrawResult)
 DynamicImage::Draw(gfxContext* aContext,
                    const nsIntSize& aSize,