Reenable background wallpapers
These got disabled in https://source.puri.sm/Librem5/phosh/commit/549da1af84010de75271e30fb8a6529ca5a1e484 and can be reenabled via #define WITH_PHOSH_BACKGROUND
. Before enabling it by default we should fix https://github.com/swaywm/wlroots/issues/897
- which we can fix now in phoc.
See also https://source.puri.sm/Librem5/phosh/issues/17 , https://source.puri.sm/Librem5/phosh/issues/6