compositor: Store original output scale

We want this so we can restore the original mode
with the original size (after having been fullscreen).
diff --git a/src/compositor.c b/src/compositor.c
index 9c0315d..db5a32d 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -2697,6 +2697,7 @@
 	output->mm_width = mm_width;
 	output->mm_height = mm_height;
 	output->dirty = 1;
+	output->origin_scale = scale;
 
 	weston_output_transform_scale_init(output, transform, scale);
 	weston_output_init_zoom(output);