magnifier: Find monitor only using mouse coordinates

There's no strict need to use the while actor position to find the sprite monitor
as eventually we care more the pointer point. So we can avoid to do some
extra computation to figure out the mouse sprite absolute positioning again.

GNOME/gnome-shell!454
4 jobs for !454 with magnifier-color-and-size in 12 minutes and 27 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #243316
check_commit_log

00:05:08

 
  Build
passed #243317
build

00:04:12

 
  Test
passed #243318
test

00:03:06

passed #243319
test-pot

00:03:06