This fixes the issue of the display freezing on reload with
wlroots#1545.
On master, all output configs are applied on reload. This may cause an
output to have its config applied up to three times, instead of just
once. The three cases are: output name, output identifier, and wildcard.
Not only is this inefficient, but it can cause swaybg to be spawned and
immediately killed.
However, swaybg requires two roundtrips of wl_display (to obtain needed
globals) before it enters its normal event loop. Modesetting will
roundtrip the wl_display. Without modesetting, waitpid for killing
swaybg could block infinitely due to swaybg being blocked by
wl_display_roundtrip.
This only configured an output once. It either uses the wildcard config
or creates an empty wildcard config and applies that. This also fixes a
bug where an output would not be reset when there is no output config to
apply to it.
272ca06171