subcompositor: consider map on creation

wl_subsurface description states:

A sub-surface becomes mapped, when a non-NULL wl_buffer is applied and
the parent surface is mapped.

Note that this doesn't require an explicit commit, which means that a
newly created subsurface with a mapped parent and a buffer already
attached must be mapped immediately. This can happen with the following
sequence of events:

- subcompositor.get_subsurface(subsurface, surface, parent)
- surface.attach(buffer)
- surface.commit()
- subsurface.destroy()
- subcompositor.get_subsurface(subsurface, surface, parent)

Fixes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3449

types/wlr_subcompositor.c

@@ -374,6 +374,8 @@ static struct wlr_subsurface *subsurface_create(struct wlr_surface *surface,
 
 	surface->role_data = subsurface;
 
+	subsurface_consider_map(subsurface, true);
+
 	return subsurface;
 }