Simplify Flip Mode rendering code with SDL_RenderCopyEx

Previously, Flip Mode rendering had to be complicated and allocate
another buffer to call FlipSurfaceVerticle, and it was just a mess.

Instead, why not just do SDL_RenderCopyEx, and let SDL flip the screen
for us? This ends up pretty massively simplifying the rendering code.

desktop_version/src/Screen.h

@@ -20,7 +20,7 @@ public:
 	void GetWindowSize(int* x, int* y);
 
 	void UpdateScreen(SDL_Surface* buffer, SDL_Rect* rect);
-	void FlipScreen(void);
+	void FlipScreen(bool flipmode);
 
 	const SDL_PixelFormat* GetFormat(void);