Since a tablet tool provides the WL_SEAT_CAPABILITY_POINTER capability,
sway will attempt to use the xcursor manager to set a cursor image. If
the tablet tool was the first (and possibly only) device to provide the
capability for the seat, the xcursor manager was not being configured
before attempting to set a cursor image. This was due to
`seat_configure_xcursor` only being called in `seat_configure_pointer`.
Since the xcursor manager was NULL in this case, it would cause a
segfault when attempting to set a cursor image. This adds a call to
`seat_configure_xcursor` in `seat_configure_tablet_tool` to ensure that
the seat has a xcursor manager.
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