Add an id field to outputs

All outputs now have a unique integer ID, allocated from a bitfield pool in the
compositor.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
diff --git a/src/compositor.c b/src/compositor.c
index 3354d21..17a0cc6 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -2365,6 +2365,9 @@
 
 	wl_list_init(&output->frame_callback_list);
 
+	output->id = ffs(~output->compositor->output_id_pool) - 1;
+	output->compositor->output_id_pool |= 1 << output->id;
+
 	output->global =
 		wl_display_add_global(c->wl_display, &wl_output_interface,
 				      output, bind_output);
@@ -2393,6 +2396,8 @@
 	wl_signal_init(&ec->unlock_signal);
 	ec->launcher_sock = weston_environment_get_fd("WESTON_LAUNCHER_SOCK");
 
+	ec->output_id_pool = 0;
+
 	if (!wl_display_add_global(display, &wl_compositor_interface,
 				   ec, compositor_bind))
 		return -1;