Implement HTMLCanvasElement.transferControlToOffscreen (#34959)

This follows the spec by introducing a new "Placeholder" canvas context
mode. The underlying offscreen canvas is kept accessible from the DOM
element to allow for the drawImage() implementation to work with canvases
that have transfered their control.

Signed-off-by: webbeef <me@webbeef.org>

components/script/canvas_state.rs

@@ -529,6 +529,15 @@ impl CanvasState {
                         smoothing_enabled,
                     ));
                 },
+                CanvasContext::Placeholder(ref context) => {
+                    context.send_canvas_2d_msg(Canvas2dMsg::DrawImageInOther(
+                        self.get_canvas_id(),
+                        image_size,
+                        dest_rect,
+                        source_rect,
+                        smoothing_enabled,
+                    ));
+                },
                 _ => return Err(Error::InvalidState),
             }
         } else {