MetaCursorTracker: Set hot spot when cursor buffer set from wl_buffer

When creating MetaCursorReference using a wl_resource, use the provided
hotspot coordinates.

This makes clients such as clickdot work more correctly.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=709593