ui/gtk-egl: fix scaling for cursor position in scanout mode

ui/gtk-egl: fix scaling for cursor position in scanout mode

vc->gfx.w and vc->gfx.h are not updated appropriately in this code path,
which leads to a different scaling factor for rendering the cursor on
some edge cases (e.g. the focus has left and re-entered the gtk window).
This can be reproduced using vhost-user-gpu with the gtk ui on the x11
backend.
Use the surface dimensions which are already updated accordingly.

Signed-off-by: Erico Nunes <ernunes@redhat.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230320160856.364319-2-ernunes@redhat.com>
(cherry picked from commit f8a951bb951140a585341c700ebeec58d83f7bbc)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>