Only consider screens to be mirrored if they are completely overlapping
Right now libnotificationmanager considers screens to be mirrored--and hence automatically enables Do Not Disturb Mode--when they are overlapping at all. But this doesn't match KScreen's definition of when screens are mirrored, which is that one screen is completely covered up by the other. Let's use that same logic here so we're not enabling DND mode when screens are simply overlapping a bit, which is a valid if unusual configuration in KScreen.
Testing done:
- Verified that DND mode is correctly enabled when two screens completely overlap
- Verified that DND mode is correctly not enabled when two screens overlap a bit