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 Name Job ID Coverage
  Build
passed build-with-xwayland #1985
librem5

00:01:29

passed build-without-xwayland #1986
librem5

00:01:36