Add support for internal screenshots

"But people already have screenshot tools", you might protest. The
rationale is simple: If you play with any video setting other than 1x
windowed (no stretching and no letterbox), then your screenshot will be
too big if you want the internal resolution of 320x240, and downscaling
will be an inconvenience.

The point is to make screenshots based off of internal resolution so
they are always pixel perfect and ideally never have to be altered once
taken.

I've added the keybind of F6 to do this.

Right now it saves to a temporary test location with the same filename;
future commits will save to properly-timestamped filenames.

desktop_version/src/GraphicsUtil.h

@@ -14,4 +14,6 @@ SDL_Color ReadPixel(const SDL_Surface* surface, int x, int y);
 void UpdateFilter(void);
 void ApplyFilter(SDL_Surface** src, SDL_Surface** dest);
 
+bool TakeScreenshot(SDL_Surface** surface);
+
 #endif /* GRAPHICSUTIL_H */