graphics optimization feature inspirated by diablo3 engine

* the rendering now consits of two panes
- the background pane (for animated stuff like the map)
- the foreground pane (for steady stuff, like UI)
each pane has it own max FPS and works idependently
this may increase graphics performance on many platforms

src/framework/graphics/texture.h

@@ -30,9 +30,11 @@ class Texture : public std::enable_shared_from_this<Texture>
 public:
     Texture();
     Texture(const ImagePtr& image);
-    Texture(int width, int height, int channels, uchar* pixels = NULL);
+    Texture(int width, int height, int channels = 4, uchar* pixels = NULL);
     virtual ~Texture();
 
+    void copyFromScreen(const Rect& screenRect);
+
     void bind();
 
     /// Tries to generate mipmaps via hardware, otherwise fallback to software implementation