swaybg: implement scaling mode "center"
swaybg: implement scaling mode "center"

colemickens/sway,1ace/sway,1ace/sway,taiyu-len/sway,crondog/sway,mikkeloscar/sway,taiyu-len/sway,christophgysin/sway,christophgysin/sway,ascent12/sway,4e554c4c/sway,sleep-walker/sway,ascent12/sway,colemickens/sway,colemickens/sway,ptMuta/sway,ascent12/sway,sleep-walker/sway,taiyu-len/sway,crondog/sway,johalun/sway,mikkeloscar/sway,4e554c4c/sway,SirCmpwn/sway,1ace/sway