bug#12956: 24.2; Move hl-line-mode to display engine so performance reason.

[Eli Zaretskii]

> From: Oleksandr Gavenko <gavenkoa@gmail.com>
> Date: Thu, 22 Nov 2012 00:17:02 +0200
> 
> >From http://debbugs.gnu.org/cgi/bugreport.cgi?bug=12927
> 
> > > Also I found that "(global-hl-line-mode 1)" slow down scrolling in CC-mode
> > > enabled buffers in two or more times. Do I need report a bug?
> 
> > No, this is expected.  hl-line-mode adds itself to post-command-hook,
> > and moves its overlay after each command that changes the current
> > line.  What this does is completely disable the redisplay
> > optimizations used when only the cursor moves without any other
> > changes to the text, because moving an overlay requires a thorough
> > redisplay of the window, which is more expensive.
> 
> > The only bug report that I can think of is a feature request to move
> > hl-line-mode into the display engine, because the display engine
> > knowns very well where's the current line (when it decides where to
> > display the cursor), and can do this highlighting better and much more
> > efficiently.

This is a duplicate of 12953.