x11: Find nearest input affecting _NET_WM_MOVERESIZE X11 requests on Wayland

The _NET_WM protocol, written before the birth of XInput 2.x, does have
no notion of different input devices whatsoever. Anyways, in a X11 session
it is safe to assume this refers about the Virtual Core Pointer since
every input device by default drives it (incl. touchscreens through the
"pointer emulating sequence", and styli).

This assumption falls apart in a Wayland session with non-pointer input,
since we do actually distinguish between all the distinct pointer devices
and touchpoints, and do not let them emulate mouse input.

We do need to specify a device/sequence there to drive the window
move/resize operation. The _NET_WM_MOVERESIZE message just gives us the
x/y root coordinates the resize was started from, so work from there
into guessing what is the most likely device/sequence that did trigger
the request on the client side.

Conversely, on Wayland we do not need to check for possible race
conditions in the pressed button states since we have larger guarantees
about not missing these events if we checked for the button modifier
mask beforehand, so make that race condition check specific to the
X11 sessions.

Closes: #2836 (closed)