bug#2530: 23/NS: redraws according to mouse-face are slow

[David Reitter]

I find that the redisplay of overlays that happens when the mouse is  
moved into an overlay with a mouse-face property are much slower in  
Emacs 23 (NS, under Cocoa/OS X).  It is pretty much a nasty animation  
- every layer is redrawn from left to right, it seems, and every step  
is visible.  It seems that background is drawn first, and then the  
text over it.

The overlays I am working with are in the header-line; I'm using (an  
adapted version of) tabbar.el for this.  The resulting package is a  
not stand-alone and I'd have a hard time turning it into a small test  
case; before I work on this, I'd rather ask here if this problem is a  
known one.

I have experimented with calls to NSDisableScreenUpdates () in  
ns_update_begin, ns_update_window_begin and ns_focus, but that didn't  
help at all.

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