internal/graphicsdriver/metal: Delay to get a texture for nextDrawable
internal/graphicsdriver/metal: Delay to get a texture for nextDrawable

See https://developer.apple.com/documentation/quartzcore/cametallayer

> To avoid stalls in your app, request a new drawable only when you
> need it, and release any references to it as quickly as possible
> after you’re done with it.

Updates #1196

hajimehoshi/ebiten,hajimehoshi/ebiten