wayland: Avoid spurious crossing events from master touch device

wayland: Avoid spurious crossing events from master touch device

Only generate crossing events on wl_touch.down for the virtual master
device used for touch events, and only whenever this virtual device
actually moves across surfaces. This behavior resembles better what is
expected in X11, where the pointer is warped to the touch position
on XITouchBegin.

This avoids the double emission of leave events when the pointer
emulating touch is lifted, that crossing event will be instead
generated when/if the focus surface changes.

https://bugzilla.gnome.org/show_bug.cgi?id=766314