Layer surface tests

This add tests for the quite central layer surface class.

since there are more changes coming up from @sebastian.krzyszkowiak in that area (which is nice) it'd be good to have these merged even though we can't run them during ci since phoc crashed there in headless mode. phoc runs fine in our smoketests so it can't be much but until then to run the tests use gtesters thorough mode:

gtester -m thorough _build/tests/test-layer-surface

the series also fixes bugs found while working on the tests.