bug#14744: 24.3.50; Flickering mouse-face on process output

[Eli Zaretskii <eliz@gnu.org>]

> From: Christopher Schmidt <christopher@ch.ristopher.com>
> Date: Sat,  3 Aug 2013 10:41:23 +0100 (BST)
> 
> > I guess one way of fixing this problem would be to modify comint.el
> > not to use overlays for this purpose.
> 
> This works around the underlying issue.  There are other real use cases
> in which, due to whatever reason, a redisplay is enforced and this
> artifact appears.

In the absence of a solution for all of them, fixing some of them
would be good nevertheless.  comint is a widely used infrastructure,
so modifying it is likely to solve quite a few of those cases.

Btw, not every case that forces a redisplay cycle actually redraws the
screen.  Emacs display engine has the last line of defense, in that,
after the display engine determines what should be on the screen, it
compares that with what actually is on the screen, and only redraws
the parts that are different.  However, places which have
mouse-highlight are always redrawn (because mouse pointer can move
asynchronously and independently of what Emacs does).