Move Graphics buffer creation to new func create_buffers()

It doesn't feel quite write leaving all the buffer creation code in
main(), even though it's perfectly okay to do so and it doesn't result
in any memory mismanagement that Valgrind can report; so I'm factoring
all of it out to a separate function, Graphics::create_buffers().

As a bonus, we no longer have to keep qualifying with `graphics.` in the
buffer creation code, which is nice.

desktop_version/src/Graphics.h

@@ -19,6 +19,7 @@ public:
 	void init();
 	void destroy();
 
+	void create_buffers(const SDL_PixelFormat* fmt);
 	void destroy_buffers();
 
 	GraphicsResources grphx;