poll the pointer location: make xeyes work

[totaam]

Some applications may use the pointer position, not just for toys like xeyes.
ie: they can use the position to place their windows or tooltips near the pointer.

This is unlikely to ever work under Wayland - "because security" (..)

Since this does expose the pointer location at all times, this should be an option, disabled by default.

[totaam]

Done in the commit above.

Usage:

XPRA_POLL_POINTER=100 xpra attach

The polling delay is in milliseconds - 100 is a good compromise.
(no packets are sent to the server if the position has not changed, and as before only the latest pointer position is sent if the queue is busy)

[totaam]

Needed a bit more testing with desktop scaling: cccb97d
(this fix should cause other problems.. hopefully)