macOS Metal | fix: wrap perform_redraw in autoreleasepool

[pcleavelin]

There seems to be a memory leak on the Metal backend during window resize where some Metal objects are not being released properly. [... release] can't be called manually on the objects because perform_redraw is called within an autorelease pool in draw_rect (since its an event handler), so that will cause a double-free. So I just wrapped perform_draw in run in an autoreleasepool and that seems to do the trick.