magnifier: Use new cursor tracker API to keep wayland focus while hidden

Since commit mutter@a2a8f0cd we force the focus surface of the meta-wayland-pointer to NULL while the pointer is hidden. This introduced an issue with the magnifier, where we use set_pointer_visible to hide the real cursor and show our own cursor at the correct position: Because the meta-wayland-pointer is still used to communicate with Wayland clients, the UI of the windows will not respond to mouse movement anymore as soon as the real cursor is hidden.

To fix this, use the newly added API of MetaCursorTracker to switch back to the old behavior while the magnifier is hiding the system cursor.

In the future and as a more correct fix, we might want to rewrite the magnifier so it doesn't have to hide the cursor and can simply show the default one (eg. by scaling the actual view instead of a clone of the view).

Related mutter#826 (closed)

Depends on mutter!832 (merged)