canvas: Deallocate WebRender images on canvas

Fixes #11062

Depends on: #11744, servo/webrender_traits#57, and servo/webrender#292

components/canvas/canvas_paint_thread.rs

@@ -706,6 +706,14 @@ impl<'a> CanvasPaintThread<'a> {
     }
 }
 
+impl<'a> Drop for CanvasPaintThread<'a> {
+    fn drop(&mut self) {
+        if let Some(ref mut wr) = self.webrender_api {
+            wr.delete_image(self.webrender_image_key.unwrap());
+        }
+    }
+}
+
 /// Used by drawImage to get rid of the extra pixels of the image data that
 /// won't be copied to the canvas
 /// image_data: Color pixel data of the image

components/canvas/webgl_paint_thread.rs

@@ -195,3 +195,11 @@ impl WebGLPaintThread {
         }
     }
 }
+
+impl Drop for WebGLPaintThread {
+    fn drop(&mut self) {
+        if let WebGLPaintTaskData::Readback(_, Some((ref mut wr, image_key))) = self.data {
+            wr.delete_image(image_key);
+        }
+    }
+}