render implement thread, wrap GL commands into tasks

src/shape.h

@@ -25,22 +25,9 @@ public:
     bool create_buffers(GLvoid* vertices, int vsize);
     void draw_fill() const;
     void draw_stroke() const;
-    void destroy()
-    {
-        if (buffers[0] || buffers[1])
-            glDeleteBuffers(2, buffers);
-#if USE_VBO
-        if (arrays[0] || arrays[1])
-            glDeleteVertexArrays(2, arrays);
-#endif // USE_VBO
-        buffers[0] = buffers[1] = 0;
-        arrays[0] = arrays[1] = 0;
-    }
+    void destroy();
     virtual bool create_attrib() { return true; };
-    ~Shape()
-    {
-        destroy();
-    }
+    ~Shape();
 protected:
     glm::vec2 quad_mid_point(glm::vec2 a, glm::vec2 b, glm::vec2 c, glm::vec2 d)
     {