WIP: Fix layer related glitches
The commit solves 2 issues: first, it makes old layers stick to where they were placed more.
Second, for the situation with only 2 layers (phosh + keyboard), where the phosh layer never goes away, it solves the refresh glitches from #15 (moved) by ensuring the keyboard layer always does the right damage when it appears.
WIP because I based it on librem5 updated with master.