Initial commit of swaybg
Initial commit of swaybg, for use with the Sway window manager.
Partially resolves T9396
Test Plan: - Use a line like output * bg /path/to/background.png in the sway config. See pretty background image.
Reviewers: Triage Team, JoshStrobl
Reviewed By: Triage Team, JoshStrobl
Maniphest Tasks: T9396
Differential Revision: https://dev.getsol.us/D10284