Upscale screenshots 2x

The plan is to have Steam screenshots always be 2x, but in the VVVVVV
screenshots directory (for F6 keybind) save both 1x and 2x.

Again, just for now, the 2x screenshot is being saved to a temporary
location for testing and will get proper timestamps later.

desktop_version/src/GraphicsUtil.h

@@ -15,5 +15,6 @@ void UpdateFilter(void);
 void ApplyFilter(SDL_Surface** src, SDL_Surface** dest);
 
 bool TakeScreenshot(SDL_Surface** surface);
+bool UpscaleScreenshot2x(SDL_Surface* src, SDL_Surface** dest);
 
 #endif /* GRAPHICSUTIL_H */