render stroke preview to image

src/node_stroke_preview.h

@@ -29,6 +29,7 @@ public:
     using parent = NodeBorder;
     static std::atomic_int s_instances;
     static std::atomic_bool s_running;
+    static std::mutex s_render_mutex;
     static std::thread s_renderer;
     static BlockingQueue<std::shared_ptr<NodeStrokePreview>> s_queue;
     static void terminate_renderer();
@@ -52,6 +53,7 @@ public:
     std::vector<StrokeFrame> stroke_draw_compute(Stroke& stroke) const;
     void draw_stroke();
     void draw_stroke_immediate();
+    Image render_to_image();
     virtual void draw() override;
     virtual void handle_resize(glm::vec2 old_size, glm::vec2 new_size, float zoom) override;
     virtual void destroy() override;