OPENGL: Add notification function about context destruction.

backends/graphics/opengl/opengl-graphics.h

@@ -136,6 +136,14 @@ protected:
 	 */
 	void notifyContextChange(const Graphics::PixelFormat &defaultFormat, const Graphics::PixelFormat &defaultFormatAlpha);
 
+	/**
+	 * Notify the manager that the OpenGL context is about to be destroyed.
+	 * This will free up/reset internal OpenGL related state and *must* be
+	 * called whenever a context might be created again after destroying a
+	 * context.
+	 */
+	void notifyContextDestroy();
+
 	/**
 	 * Adjust the physical mouse coordinates according to the currently visible screen.
 	 */