Flickering on Metal with Immediate presentation mode

[kvark]

Dota shows visible artifacts in refreshed frames, but only when running with v-sync OFF.

I suspect the problem coming from this piece:

msg_send![drawable, present];
msg_send![drawable, release];

When running with v-sync, present() blocks until the vertical ray, then displays the contents, and then returns. So we are free to release the drawable right after.
When running without v-sync, the present() returns immediately, but there is still going to be some time before the image appears on screen. We need to hold it for the mean time and not release.

[kvark]

Fixed by #2169