don't use the xdg window geometry offset as a position

It is the offset of the visible bounds inside the surface, which is not
zero for clients that pad their surface or place subsurfaces outside it,
so floating windows were mapped offset by their client side decorations.

client.h

@@ -189,6 +189,7 @@ client_get_geometry(Client *c, struct wlr_box *geom)
 	}
 #endif
 	*geom = c->surface.xdg->geometry;
+	geom->x = geom->y = 0;
 }
 
 static inline Client *