Save energy when the window is minimized

[Dawoodoz]

This would allow pausing and minimizing a game on a laptop without wasting battery on rendering images that are not displayed. The hard part is to not complicate the main loop too much, while still giving enough manual control over resources.

This only affects applications that render continuously and allow leaving full-screen, because waiting for an event before drawing will stay idle when minimized by not getting events.

[Dawoodoz]

A boolean function can be used to ask if the window is potentially visible, which can default to true when not implemented. When false, the calling application can pause logic and skip rendering.