Fix SDLTexture backend's linked-list

Was causing some use-after-free behaviour, according to valgrind.

Should fix #87

src/Backends/Rendering/SDLTexture.cpp

@@ -116,6 +116,9 @@ Backend_Surface* Backend_CreateSurface(unsigned int width, unsigned int height)
 	surface->next = surface_list_head;
 	surface_list_head = surface;
 
+	if (surface->next != NULL)
+		surface->next->prev = surface;
+
 	return surface;
 }