Suspend execution when Eternity is off screen

[ioan-chera]

Eternity should pause running everything unnecessary when off screen (minimized, behind other windows or on another virtual desktop). This is very important when playing it from a battery powered laptop and in some cases, if the laptop is silent and efficient, it's easy to forget. Right now it still runs rendering even if unnecessary, in situations such as the game being paused.

[ioan-chera]

So far, disabled D_Display when window is fully occluded (without necessarily being minimized). But only on some systems

• macOS
• Windows (maybe this? https://stackoverflow.com/a/34194313)
• Linux

May also need to think about disabling sounds, autopausing etc. (sometimes we don't want to autopause, such as when playing or especially testing demo).