recorder: Hide cursor while magnifier is active

The magnifier adds its own copy of the system cursor to apply the
expected transformations, so we don't need to add it again in the
recorder; this avoids two different cursors showing up in recordings,
but doesn't address the case where the cursor should not be recorded
at all, but the magnifier adds it anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=700488