The legacy fullscreen state needs to be detected at MapRequest time,
because that is when the X11 client has alredy set up the initial window
state.

Doing it at xserver_map_shell_surface() meant that it would be done as a
response to Xwayland creating the wl_surface and XWM receiving the
WL_SURFACE_ID ClientMessage, whichever came later. At that point the X11
client might still be setting things up in theory, though in practice
most of the X11 communication has already happened when
xserver_map_shell_surface() gets called.

The real reason for this is to clean up xserver_map_shell_surface() from
everything that would affect drawing the decorations. This patch is one
part of that clean-up.

The weston_output_weak_ref logic is not put into compositor.h, because
there are no other users for it at this time. We need to protect against
the output going away.

A side-effect of this patch is that saved_width and saved_height will
now get overwritten also for legacy fullscreen windows. Previously, they
were left to zero as far as I could tell.

NOTE: This stops override-redirect legacy fullscreen windows from being
detected as fullscreen. MapRequest processing does not happen for OR
windows. These windows get detected as type XWAYLAND instead.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>