Improve MPX interaction with drag-and-drop

Improve MPX interaction with drag-and-drop

* src/xfns.c (Fx_set_mouse_absolute_pixel_position): Use
internal client pointer record.
* src/xterm.c (x_dnd_cancel_dnd_early): New function.  Only used
on XI2 builds so far.
(x_dnd_begin_drag_and_drop): Set the pointer device used for DND
events.
(xi_disable_devices): Cancel the drag-and-drop operation if that
device is disabled.
(x_send_scroll_bar_event): Update outdated comment.
(handle_one_xevent): Only accept DND events from that device.
(frame_set_mouse_pixel_position): Use internal client pointer
record.