Das klappt wunderbar, danke! Problem gelöst.

EDIT: Falls folgendes legitim ist:

if(old_screen_size_x != screen_size.x) {
old_screen_size_x = screen_size.x;
if(render_target) ptr_remove(render_target);
render_target = bmap_createblack(screen_size.x,screen_size.y,32);
}