xdg-shell: Specify the meaning of 0x0 window geometry in configure

Some times the compositor needs to send a configure request but without
having any clue about what size the surface should have. Examples
include unmaximizing a surface that was mapped as maximized, or an
initial state which doesn't have any size expectations.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

protocol/xdg-shell.xml

@@ -317,6 +317,11 @@
 	about how its surface should be resized in window geometry
 	coordinates. See set_window_geometry.
 
+	If the width or height arguments are zero, it means the client
+	should decide its own window dimension. This may happen when the
+	compositor need to configure the state of the surface but doesn't
+	have any information about any previous or expected dimension.
+
 	The states listed in the event specify how the width/height
 	arguments should be interpreted, and possibly how it should be
 	drawn.