Convert all scales to int32_t

The type changed in the protocol, so update weston for this.
diff --git a/clients/window.h b/clients/window.h
index f7b3f28..476d284 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -258,7 +258,7 @@
 
 void
 window_set_buffer_scale(struct window *window,
-                        uint32_t scale);
+                        int32_t scale);
 
 uint32_t
 window_get_output_scale(struct window *window);