Added X11 support

@trimental Thanks for the PR, but I'd suggest to hold until #6079 since it contains all the changes required for upstream winit. I can apply your patch later, since we'll have rendering related patches as well and they affect the should_draw logic.

FYI: Looks like Occluded also means when another window is above your window. I was checking the docs, but I may be wrong, so please correct me then.

Now, from first sight, it might not make sense to render. But what iff the window that's occluding you is actually semi-transparent. Then you'd show stale content. Having semi-transparent windows in 2022 isn't actually a too rare condition, because users seem to like that and configure their desktop as such (e.g. for file managers, or actually terminals, with and without blur effect).

If the window is actually hidden/symbolized, such an optimization makes 100% sense, but the occlusion due to other windows laying on top of you, I fear will generate confusion.

p.s.: forgot to mention, even when symbolized/hidden, the window might need to be rendered up-to-date because desktop's like to show you a preview when Alt Tab'ing through the windows, or even when simply hovering the mouse over the app window of a symbolized/hidden window. Can such an event be distinguished from? If not, I'd even say the whole idea is sadly not applicable.

@christianparpart Yes you're completely right in all of those assumptions, ie we still need to render when the compositor is presenting previews or behind semi transparent windows.

On macOS both of these are 100% taken into account, so the window is only marked as occluded if it completely cannot be seen in any way. On x11 its a bit harder to know whether all compositors will remember to take care of this, but considering the x11 docs.

When the window changes state from viewable and completely unobscured, viewable and partially 
obscured, or not viewable to viewable and fully obscured, the X server generates the event with the 
state member of the XVisibilityEvent structure set to VisibilityFullyObscured.

I would definitely interpret that to mean all x11 compositors should account for transparency and previewing, although I haven't fully tested different X11 compositors

@trimental the winit patch got merged. So you can rebase.

@kchibisov done :)