implement alpha locking per layer

engine/canvas.h

@@ -13,7 +13,7 @@ class Layer
 public:
     RTT m_rtt[6];
     bool m_visible = true;
-    bool m_locked = false;
+    bool m_alpha_locked = false;
     float m_opacity = 1.f;
     std::string m_name;
     struct Snapshot
@@ -41,6 +41,7 @@ class Canvas
 public:
     static Canvas* I;
     bool m_erase = false;
+    bool m_alpha_lock = false;
     glm::mat4 m_mv;
     glm::mat4 m_proj;
     glm::vec4 m_box;
@@ -63,6 +64,7 @@ public:
     static glm::mat4 m_plane_transform[6];
     Sampler m_sampler;
     Sampler m_sampler_bg;
+    Sampler m_sampler_mask;
     glm::vec2 m_cam_rot;
     float m_cam_fov = 85;