Creates an RTImage cache

Fixes:
- Fixes GUI issue on Windows (GDI handles limit reached)
- Fixes an incorrect icon name in Perspective tool
- Adds robustness RTScalable::loadSurfaceFromIcon function

rtgui/rtsurface.h

@@ -52,4 +52,5 @@ public:
     bool hasSurface();
 
     Cairo::RefPtr<Cairo::ImageSurface> get();
+    void updateSurface();
 };