render merged layers cache

src/canvas.h

@@ -167,6 +167,7 @@ public:
     std::unique_ptr<Stroke> m_dual_stroke;
     bool m_show_tmp = false;
     std::vector<std::unique_ptr<Layer>> m_layers;
+    Layer m_layers_merge;
     std::vector<int> m_order;
     std::vector<glm::vec2> m_plane_shape[6]; // screen space projection of the plane
     glm::mat4 m_plane_unproject[6] = SIXPLETTE(glm::mat4(1));
@@ -184,6 +185,8 @@ public:
     Texture2D m_brush_mix;
     Texture2D m_tex[6];
     Texture2D m_tex2[6];
+    RTT m_merge_rtt;
+    Texture2D m_merge_tex;
     bool m_pick_ready[6];
     std::unique_ptr<glm::u8vec4[]> m_pick_data[6] = SIXPLETTE(nullptr);
     static glm::vec3 m_plane_origin[6];
@@ -247,6 +250,7 @@ public:
     void stroke_end();
     void stroke_cancel();
     void stroke_commit();
+    void draw_merge(std::array<bool, 6> faces = SIXPLETTE(true));
     void clear(const glm::vec4& color = { 1, 1, 1, 0 });
     void clear_all();
     void pick_start();