Magnifier sprite scaling fixes
Supporting multi-DPI setups, depends on !454 (merged).
Solution is not optimal because we'd probably need some new API in mutter, but ideally I'd like to have this in before for having in 3-32 branch, and prepare a better fix (needed also for screencasting/shotting and fixing #1030).
Before the scaling changes: Screencast_12-03-2019_12_32_01