Fix tooltips flickering on X during `mouse-drag-and-drop-region'

* src/xterm.c (x_get_window_below): New function.
(XTmouse_position): If track-mouse is drag-source and a window
is a tooltip, look below it.
(handle_one_xevent): Don't generate a nil help-echo event on
LeaveNotify if the display is grabbed and track-mouse is
drag-source.
2 jobs for master in 56 minutes and 28 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build Images
failed #46638
build-image-inotify

00:56:28

 
  Normal
skipped #46639
test-src-inotify
 
Name Stage Failure
failed
build-image-inotify Build Images
executor failed running [/bin/sh -c make bootstrap]: exit code: 2
Running after_script
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
$ find ${test_name} ! \( -name "*.log" -o -name ${EMACS_TEST_JUNIT_REPORT} \) -type f -delete
find: build-image-inotify-2a40fa3d: No such file or directory
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1