cursor-tracker: Add API to keep the wayland pointer focus while hidden

Since commit a2a8f0cd we force the focus surface of the meta-wayland-pointer to NULL while the pointer is invisible. This introduced an issue with the a11y magnifier of the shell, which uses meta_cursor_tracker_set_pointer_visible to hide the real cursor and show its own magnified 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.

Fix this issue for now by adding an additional method to the cursor-tracker which allows disabling the behavior commit a2a8f0cd introduced.