set_cloexec is defined by both sway and wlroots (and who-knows-else),
so rename the sway one for supporting static linkage. We also remove
the duplicate version of this in client/.
Fixes: https://github.com/swaywm/sway/issues/4677
The new upstream is https://github.com/swaywm/swaybg
This commit also refactors our use of gdk-pixbuf a bit, since the only
remaining reverse dependency is swaybar tray support.
In addition to removing unused code, two minor problems are fixed:
(1) `resize set` and `resize adjust` did not error when given
too many arguments.
(2) `orientation` was incorrectly overridden to be 'U' for
scroll events in the swaybar tray `handle_click` function.
