add glad to load OpenGL extensions

src/brush.h

@@ -136,19 +136,6 @@ struct StrokeSample
     }
 };
 
-class BrushMesh
-{
-public:
-    GLuint buffers[3]{ 0 };
-    GLuint vao{ 0 };
-    struct instance_t { glm::mat4 mvp; float flow; };
-    int loc_flow = 0;
-    int loc_mvp = 0;
-    
-    bool create();
-    void draw(const std::vector<StrokeSample>& samples, const glm::mat4& proj);
-};
-
 class Stroke
 {
 public: