xfwm: Setting `--vblank=off` as a user

[Olivier Fourdan]

Hi,

With
```
$ xfwm4 --version
This is xfwm4 version 4.13.1 (revision 942156e4) for Xfce 4.12

So you're build predates and does not feature the fix for GL vblank
(commit 0e6b940/bug #14564)

Released under the terms of the GNU General Public License.
Compiled against GTK+-3.22.30, using GTK+-3.22.30.
- Startup notification support: Yes
- XSync support: Yes
- Render support: Yes
- Xrandr support: Yes
- Xpresent support: No

And your build is incomplete, Xpresent should be the default but it's
not enabled in your build.

- Embedded compositor: Yes
- Epoxy support: Yes
- KDE systray proxy (deprecated): No
```
one of our users has an issue with the amdgpu graphics driver.

So you're defaulting to GL for vblank which is broken in the version you use.

`xfwm4 --replace --vblank=off` works around that problem, so we would
like to disable it, on only that system, and it’d be nice, if the
users could do it themselves.
Reading [1], it should be possible, but there is no option
*Synchronize drawing to the vertical blank* in the tab *Compositor*
in xfwm4-tweaks-settings.
I am now going to configure it in `/etc/Xmor erecent11/xorg.conf.d/`, but it’d
be great if the user could do it.

No, this is not meant to be a user option - Having an option to
"unbreak things" is usually suboptimal, I would advise to 1. enable
Xpresent in your build and 2. use a more current source tree for your
build.

Cheers,
Olivier