Add Graphics::destroy_buffers()

These destroy all the buffers that are created on the Graphics class.
Since these buffers can't be created at the same time as the rest of
Graphics is (due to the fact that they require knowing the pixel format
of the game screen), they can't be destroyed at the same as the rest of
Graphics is, either.

desktop_version/src/Graphics.h

@@ -19,6 +19,8 @@ public:
 	void init();
 	void destroy();
 
+	void destroy_buffers();
+
 	GraphicsResources grphx;
 
 	int bfontlen(uint32_t ch);