Cocoa: refactor commit 876a428f.

Conditions for when updateExposedGeometry() should
actually send the expose event goes into the function
itself.

The window()->isVisible() check could arguably be
moved to isWindowExposable(), but I'm keeping this
as a straight refactor without any behavior changes.
(isWindowExposable() is called from multiple locations)

Change-Id: I6d792286ccbc50065ebfc588daca2240761a1937
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>