Update WR (opaque rect optimization, stacking context culling).

Correct viewport test references

These references were using an incorrect value for the expected
container width. Now that WebRender is no longer clipping to stacking
context boundaries the rendered width is correct (100% of viewport
width = 240px).

components/canvas/webgl_paint_thread.rs

@@ -261,7 +261,8 @@ impl WebGLPaintThread {
                                                offset: 0,
                                                is_opaque: false,
                                            },
-                                           pixels.clone());
+                                           pixels.clone(),
+                                           None);
 
                 let image_data = CanvasImageData {
                     image_key: image_key,