Change retrieval of buffers from sublayers to both clip relative

to the scrolled position of its parent layer and to offset the
origin, since each layer assumes that its origin is 0,0.

src/components/main/compositing/compositor_layer.rs

@@ -244,10 +244,15 @@ impl CompositorLayer {
         let transform = |x: &mut CompositorLayerChild| -> bool {
             match x.container.scissor {
                 Some(scissor) => {
-                    let new_rect = window_rect.intersection(&scissor);
+                    let new_rect = rect.intersection(&scissor);
                     match new_rect {
                         Some(new_rect) => {
-                            x.child.get_buffer_request(new_rect, scale)
+                            // Child layers act as if they are rendered at (0,0), so we
+                            // subtract the layer's (x,y) coords in its containing page
+                            // to make the child_rect appear in coordinates local to it.
+                            let child_rect = Rect(new_rect.origin.sub(&scissor.origin),
+                                                  new_rect.size);
+                            x.child.get_buffer_request(child_rect, scale)
                         }
                         None => {
                             false //Layer is offscreen