compositor: only reflow the outputs if the shell did not move them

weston_compositor_reflow_outputs() assumes that all output are positioned from
left to right with no gaps in the same order in which they where created.

If the shell moves an output with weston_output_move() then this assumption is
no longer true. So stop reflowing the outputs in the case. The shell is now
responsible for positioning all outputs as needed.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>

include/libweston/libweston.h

@@ -1263,6 +1263,7 @@ struct weston_compositor {
 	struct wl_list plugin_api_list; /* struct weston_plugin_api::link */
 
 	uint32_t output_id_pool;
+	bool output_flow_dirty;
 
 	struct xkb_rule_names xkb_names;
 	struct xkb_context *xkb_context;