Disable compositing WM in MATE and XFCE by default

Many older devices (e.g. Nvidia Tegra tablets) use the XFCE and MATE desktop environments to cope with low resources.

However, the default choice of leaving the compositing WM enabled seems to have some significant performance overhead when many windows are open.

I was wondering whether it made sense to disable the compositing WM preference in both postmarketos-ui-mate and postmarketos-ui-xfce4 by default, since it can still be re-enabled easily in the WM preferences tab.