implement layer bounds optminize function, add history to draw_objects with the right bounds.

src/rtt.h

@@ -17,10 +17,12 @@ public:
     ~RTT();
 
     void destroy();
+    void copy(const RTT& source);
     void resize(int width, int height);
     bool create(int width, int height, int tex = -1, GLint internal_format = GL_RGBA8);
     bool recreate() { return create(w, h); }
     void clear(glm::vec4 color = glm::vec4(0));
+    glm::ivec4 calc_bounds();
     uint8_t* readTextureData(uint8_t* buffer = nullptr);
     float* readTextureDataFloat(float* buffer = nullptr);
     uint8_t* createBuffer();