Fix for a combobox. The problem is quite subtle and tough - with Cocoa I have to

process "paint events" immediately, but ROOT also can draw outside of paint event.
And with combobox there are 2 "rendering passes", one is with incorrect sizes, the second one 
is with correct sizes. With X11/Win32 these passes are sequenced - and the second
one is initiated by expose event. With Cocoa the second pass actually happens before the 
first one. This, plus some implementation details and different event order results
in quite ugly visual artefact - the button in the right part of combobox is 
not correctly repainted.


git-svn-id: http://root.cern.ch/svn/root/trunk@45132 27541ba8-7e3a-0410-8455-c3a389f83636