bug#16594: 24.3.50; very slow redraw when resizing windows horizontally

[martin rudalics]

> emacs -q
> (scroll-bar-mode 'nil)
> C-x 3
>
> then try to resize the windows using mouse several times and emacs
> redisplay is so slow that you can notice the fringes besides the drag
> handle is repainted from top to bottom frame by frame.
>
> profiling shows `read-event' takes much of the time, is this the real
> cause?
>
> - command-execute                                                 207  98%
>  - call-interactively                                             207  98%
>   - mouse-drag-vertical-line                                      202  96%
>    - mouse-drag-line                                              202  96%
>     - eval                                                        202  96%
>      - track-mouse                                                202  96%
>       - funcall                                                   202  96%
>        - #<compiled 0x40a712ed>                                   202  96%
>         - read-event                                              184  87%
>          - redisplay_internal (C function)                          1   0%
>             eval                                                    1   0%
>   + profiler-report                                                 5   2%
> - ...                                                               2   0%
>    Automatic GC                                                     2   0%
> + redisplay_internal (C function)                                   1   0%

I checked in a fix such that mouse dragging a line doesn't occur
pixelwise by default.  Does this improve the behavior?  I have no idea
how to interpret the profiler report - when you drag a line it will
always be the case that most of the cpu is consumed by that.

martin