implement the complete stroke shader with custom blending mode

engine/canvas.h

@@ -17,11 +17,14 @@ public:
     int m_width;
     int m_height;
     bool m_use_instanced = false;
+    bool m_show_tmp = false;
     std::vector<Layer> m_layers;
     std::vector<Stroke> m_strokes;
-    std::unique_ptr<Layer> m_tmp;
+    RTT m_tmp;
     RTT m_fb;
+    Texture2D m_tex;
     Sampler m_sampler;
+    Sampler m_sampler_bg;
 
     bool create(int width, int height);
     void resize(int width, int height);
@@ -30,6 +33,7 @@ public:
     void stroke_update(glm::vec2 point, float pressure);
     void stroke_draw();
     void stroke_end();
+    void stroke_commit();
     void clear();
 };