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/core/clock.h

@@ -35,7 +35,7 @@ public:
 
     ticks_t asyncTicks();
     ticks_t ticks() { return m_currentTicks; }
-    ticks_t ticksElapsed(long prevTicks) { return m_currentTicks - prevTicks; }
+    ticks_t ticksElapsed(ticks_t prevTicks) { return m_currentTicks - prevTicks; }
     ticks_t ticksFor(int delay) { return m_currentTicks + delay; }
 
     float asyncTime() { return asyncTicks()/1000.0f; }