Implement scaling modes manually

For future PRs, it'll be very nice to have full control over how VVVVVV
gets drawn to the window. This means we can use the entire window size
for things like touch input, drawing borders, or anything we want.

desktop_version/src/Graphics.h

@@ -258,6 +258,10 @@ public:
     int screenshake_x;
     int screenshake_y;
 
+    void draw_window_background(void);
+
+    void get_stretch_info(SDL_Rect* rect);
+
     void render(void);
     void renderwithscreeneffects(void);
     void renderfixedpre(void);