libweston: Add view unmap listener to pointer constraints

Since the logic of pointer constraints assumes a valid view throughout, add a
signal to disable constraints when its current view is unmapped by Weston.

The assumption that a previously unmapped view is valid already leads to the
constraints code crashing. This can happen when attaching a NULL buffer to the
surface and commiting, which effectively unmaps the view with the side effect of
clearing the surface's input region, which is then assumed valid inside
maybe_warp_confined_pointer().

Fixes: #721

Signed-off-by: Sergio Gómez <sergio.g.delreal@gmail.com>