rootston: Keep older anchored layers near anchor points

Layer surfaces are attached to edges of the screen starting with the youngest, causing new ones to always displace existing ones. This changes the order to oldest first, keeping the positions more often.

Copy of upstream's https://github.com/swaywm/wlroots/pull/1239
2 jobs for f/from-upstream in 3 minutes and 5 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #1985
librem5
build-with-xwayland

00:01:29

passed #1986
librem5
build-without-xwayland

00:01:36