Add magnifier for rectangle mode
Summary:
Add magnifier for rectangle mode, it can show 5x of the mouse pointer
Depends on D11598
FEATURE: 392482
Test Plan:
In rectangle mode:
- select rectangle, check magnifier
- adjust selection, check magnifier
- change the selection to any size and check magnifier
- test magnifier on multi-screen
- test magnifier in HiDPI mode
Reviewers: Spectacle, rkflx, ngraham, alexeymin
Reviewed By: Spectacle, rkflx, ngraham, alexeymin
Subscribers: alexeymin, rkflx, ngraham
Differential Revision: https://phabricator.kde.org/D11599