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

[Oleksandr Gavenko]

>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.

-- 
Best regards!