compositor: note, weston_surface_damage does it wrong

The fix is not trivial, so I want to document the problem before I
forget about it again.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
diff --git a/src/compositor.h b/src/compositor.h
index c942365..76af451 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -871,7 +871,10 @@
 	struct wl_resource *resource;
 	struct wl_signal destroy_signal;
 	struct weston_compositor *compositor;
+
+	/** Damage in local coordinates from the client, for tex upload. */
 	pixman_region32_t damage;
+
 	pixman_region32_t opaque;        /* part of geometry, see below */
 	pixman_region32_t input;
 	int32_t width, height;