clutter/x11: Don't set the stage on crossing events during implicit grabs

On x11 we emulate pointer events from touch events as long as there's
only one touchpoint on screen, this obviously leads to x11 sending us
crossing events triggered by the emulated pointer. Now if we get a leave
event and set the stage of the ClutterInputDevice to NULL, new touch
events will be discarded by clutters backend because the core pointer
doesn't have a stage associated. This means Mutter completely loses
state of a touchpoint as soon as it crosses a shell actor.

As a solution, don't update the stage of the input devices on crossing
events that happen during the pointer is implicitly grabbed.

GNOME/mutter!423
5 jobs for !423 with clutter-fix-x11-backend-bug in 20 minutes and 15 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Review
passed #369850
check-commit-log

00:00:40

 
  Build
passed #369851
build-mutter

00:16:05

passed #369852
build-without-native-backend

00:15:11

 
  Test
passed #369854
can-build-gnome-shell

00:02:05

passed #369853
test-mutter

00:03:28