implement simple brush projection on single cube face, add camera rotation instead of pan

engine/canvas.h

@@ -11,11 +11,13 @@ NS_START
 class Canvas
 {
     Plane m_plane;
+    Plane m_plane_brush;
     BrushMesh m_mesh;
     bool m_dirty = false;
 public:
     bool m_erase = false;
-    glm::mat4 m_mvp;
+    glm::mat4 m_mv;
+    glm::mat4 m_proj;
     glm::vec4 m_box;
     int m_width;
     int m_height;
@@ -30,6 +32,8 @@ public:
     Texture2D m_tex2;
     Sampler m_sampler;
     Sampler m_sampler_bg;
+    glm::vec2 m_cam_rot;
+    float m_cam_fov = 85;
 
     bool create(int width, int height);
     void resize(int width, int height);