Replaced with draw_all() that already has autorelease pool around it

[piotrkundu]

#98

[wjakob]

Hi Piotr,

draw_all() of course subsumes all this, but the point of this example was to show a bit more how a larger portion of the framework setup/draw calls can be moved over to a hand-written main loop. Perhaps you could change the PR to simply a comment that says

/* Alternatively call 'screen->draw_all();' that subsumes all of the above steps,
   including setting up an autorelease pool on macOS / Metal targets */