This allows setting window specific images.
Also Changed:
- The colored background is _always_ drawn, to prevent non-fullscreen
images from leaking information.
- Added function list_arbitrary_insert, which allows insertion to a
list at an arbitrary location less than the capacity. It silently
fails otherwise.
Calling `exit` in sway_terminate prevents sway from correctly shutting
down (freeing data, cleanly terminating the ipc server, etc.).
A better way is to exit straight away if the failure occurs before
`wlc_run` and use sway_abort as usual if it occur when wlc is running.
There is only a slight issue. When using a transparent color the views
are arranged to make room for swaylock which we can now see. I tried removing
the arrange call but that just made it worse by putting in an opaque view on the
workspace and not making the lockoverlay color. Ill raise an issue if
this is not easily solved