1) add Wayland code that prevent subwindows from leaking outside their parent.
This does not cover GL subwindows.
2) add macOS code that prevent GL subwindows from leaking outside their parent.
This fixes issue #494 for the macOS platform.
N.B.: Wayland GL subwindows are not prevented from leaking because no solution
that would not require any change in client applications was found. Code that
would cover Wayland GL subwindows but would require client applications to always
use the FL_ALPHA flag is included in this commit in commented out form.
