Extract canvas stroke runtime and preview draw shells

src/legacy_node_canvas_draw_services.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include "node_canvas.h"
+
+namespace pp::panopainter {
+
+void init_legacy_node_canvas_shell(NodeCanvas& node_canvas);
+void draw_legacy_node_canvas_shell(NodeCanvas& node_canvas);
+
+} // namespace pp::panopainter