add support for high res brush textures, implement mipmaps on brush

engine/canvas.h

@@ -74,6 +74,7 @@ public:
     static glm::vec3 m_plane_tangent[6];
     static glm::mat4 m_plane_transform[6];
     Sampler m_sampler;
+    Sampler m_sampler_brush;
     Sampler m_sampler_bg;
     Sampler m_sampler_mask;
     glm::vec2 m_cam_rot;