kcm: fix more papercuts with screen mirroring

Xaver Huglrequested to merge
work/zamundaaa/more-papercuts into master

Commit 1 is just a refactor / bugfix for edge cases

Commit 2 prevents the user from mirroring a disabled screen, which would get the KCM all confused and buggy

Commit 3 makes changing the scale on replication sources work properly, without any weird warnings about gaps or overlaps between outputs

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