magnifier: Find monitor only using mouse coordinates

There's no strict need to use the whole 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!475
4 jobs for !475 with magnifier-sprite-scaling in 6 minutes and 11 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Review
passed #286839
check_commit_log

00:01:34

 
  Build
passed #286840
build

00:03:08

 
  Test
passed #286841
test

00:01:28

passed #286842
test-pot

00:01:27