Fix the issue of lost proximity events when proximity occurs on a surface that is not bound to the tablet protocol
https://invent.kde.org/aki1/kwin/-/blob/master/src/wayland/tablet_v2.cpp#L354 https://invent.kde.org/aki1/kwin/-/blob/master/src/wayland/tablet_v2.cpp#L273
If we miss storing m_lastTablet with sendProximityIn, the value becomes nullptr. As a result, windows bound to the tablet protocol become unresponsive to tablet events until we move stylus away from tablet and approach it again.
Edited by Aki Sakurai