libweston-desktop/xwayland: Handle correctly a transition from fullscreen

As an older geometry is alway set 0 when coming back from fullscreen
state, avoid computing newer offset values based on it as that would
cause an incorrect xwayland surface movement, equivalent to the size of
the theme's setup-up margin.

Fixes: #454

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
5 jobs for adjust-xwayland-offset in 3 minutes (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Container Prep
passed #9542367
container_prep

00:00:14

 
  Build
passed #9542369
kvm
build-docs

00:00:54

passed #9542368
kvm
build-native-meson-default-options

00:02:32

passed #9542370
kvm
build-native-meson-no-gl-renderer

00:01:04

 
  Pages
passed #9542371
docs-and-coverage

00:00:13