Implement Infrastructure to reuse previous frames for brush drawing

Implement Infrastructuro to reuse the previous evaluation of the
node graph to blend the new stroke with instead of drawing the
entire trace from scratch.
This does not transition to a blending based approach because that still
caused regressions but allows the brush node to work with input data
natively.

Test Plan:
- Use the brush tool in the editor and check for regressions
- Evaluate the performance

Reviewers: Keavon

Pull Request: https://github.com/GraphiteEditor/Graphite/pull/1190

frontend/wasm/src/editor_api.rs

@@ -708,21 +708,26 @@ impl JsEditorHandle {
 
 	/// Returns the string representation of the nodes contents
 	#[wasm_bindgen(js_name = introspectNode)]
-	pub fn introspect_node(&self, node_path: Vec<NodeId>) -> Option<String> {
+	pub fn introspect_node(&self, node_path: Vec<NodeId>) -> JsValue {
 		let frontend_messages = EDITOR_INSTANCES.with(|instances| {
 			// Mutably borrow the editors, and if successful, we can access them in the closure
 			instances.try_borrow_mut().map(|mut editors| {
 				// Get the editor instance for this editor ID, then dispatch the message to the backend, and return its response `FrontendMessage` queue
-				editors
+				let image = editors
 					.get_mut(&self.editor_id)
 					.expect("EDITOR_INSTANCES does not contain the current editor_id")
 					.dispatcher
 					.message_handlers
 					.portfolio_message_handler
-					.introspect_node(&node_path)
+					.introspect_node(&node_path);
+				let image = image?;
+				let image = image.downcast_ref::<graphene_core::raster::ImageFrame<Color>>()?;
+				let serializer = serde_wasm_bindgen::Serializer::new().serialize_large_number_types_as_bigints(true);
+				let message_data = image.serialize(&serializer).expect("Failed to serialize FrontendMessage");
+				Some(message_data)
 			})
 		});
-		frontend_messages.unwrap()
+		frontend_messages.unwrap().unwrap_or_default().into()
 	}
 }