compositor: Don't hardcode output transform when sending the event

We add a transform field to weston_output and backends can override it
when they create the output.
diff --git a/src/compositor.h b/src/compositor.h
index b3c554b..ccac91e 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -168,6 +168,7 @@
 
 	char *make, *model;
 	uint32_t subpixel;
+	uint32_t transform;
 	
 	struct weston_mode *current;
 	struct weston_mode *origin;