This adds handling of UnmapNotify / MapNotify / VisibilityNotify to
track whether windows are visible. When hidden, the refresh loop is
halted until visible again. Often these refreshes were just checking if
the window is dirty, but I believe it sometimes did a full re-render for
things that change without user interaction (cursor blink, animations).

This also changes handling of Expose events to set a flag indicating the
next refresh should have `require_presentation: true`.

Release Notes:

- x11: No longer refreshes windows that aren't visible.