Selection issue on OS X

Howdy!

Selection isn't working properly on OS X, as shown in
http://bugzilla.abisource.com/show_bug.cgi?id=13116
Trying to solve that bug, I found out that selection, per si, was
working (almost) properly, just doesn't drawing the selection.

One way to solve it, was force a _generalUpdate() on _drawSelection()
function, but I don't know about solution's correctness.

I attached the patch in http://bugzilla.abisource.com/show_bug.cgi?id=13116#c1

Could someone review?

Best Regards,

-- 
Fabiano Fidêncio