display: Try harder to select a good and working scale

When swicthing configurations, we should ensure that the selected scale is valid. This is complicated by the fact that it depends on other factors whether we need a single global scale or not.

Try to retain the scale that monitors have if possible. If that does not work, try to use the old scale of the primary monitor everywhere. This should result in a good behaviour in most situations.

Co-authored-by: Stéphane Travostino steph@combo.cc

Fixes: #1038 (closed)