move the ui/shaders reload into the ui thread, add rtt region copy, add box/rect conversion

src/rtt.h

@@ -19,7 +19,10 @@ public:
     ~RTT();
 
     void destroy();
+    // copy with interpolation
     void copy(const RTT& source);
+    // copy a region
+    void copy(const RTT& source, const glm::vec4& rect);
     void resize(int width, int height);
     bool create(int width, int height, int tex = -1, GLint internal_format = GL_RGBA8, bool depth_buffer = false);
     bool recreate() { return create(w, h); }