ui/gtk: Use consistent naming for variables in different coordinates

ui/gtk: Use consistent naming for variables in different coordinates

Now that we've documented definitions and presentation of various
coordinates, let's enforce the rules.

Signed-off-by: Weifeng Liu <weifeng.liu.z@gmail.com>
Message-ID: <20250511073337.876650-3-weifeng.liu.z@gmail.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
(cherry picked from commit 3a6b314409b42fe7c46c2bd80cfc2a6744d414fe)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>