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.h b/src/compositor.h
index b19900b..0ef342b 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -73,6 +73,8 @@
 };
 
 struct weston_output {
+	uint32_t id;
+
 	struct wl_list link;
 	struct wl_global *global;
 	struct weston_compositor *compositor;
@@ -240,6 +242,8 @@
 
 	struct screenshooter *screenshooter;
 	int launcher_sock;
+
+	uint32_t output_id_pool;
 };
 
 #define MODIFIER_CTRL	(1 << 8)