We used to "invalidate" scroll valuators, so the next scroll event could
be used as the base for the next scroll deltas. This has the inconvenience
that it invariably consumes the first event received after enter and,
due to interactions with WM overeager passive button grabs, there's a
possibility we don't scroll at all if we receive interleaved "smooth
scroll" XI_Motion events and XI_Enter events (Normally triggered by regular
scroll wheels in mice).

In order to fix this, and at the expense of some sync-call overhead on
XI_Enter events (one XIQueryDevice call per slave device), query the
current scroll valuator state for all the slaves of the entered pointer,
so we do know beforehand the right base values. If new devices are plugged
while the pointer is on top of the client, the initialized scroll values
will match the valuators'.

https://bugzilla.gnome.org/show_bug.cgi?id=750994
https://bugzilla.gnome.org/show_bug.cgi?id=750870