window: Add window_get_output_scale()

This lets you find the maximal scale for all the outputs a window
is on, which is useful for picking a buffer_scale.
diff --git a/clients/window.h b/clients/window.h
index fec601f..f7b3f28 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -260,6 +260,9 @@
 window_set_buffer_scale(struct window *window,
                         uint32_t scale);
 
+uint32_t
+window_get_output_scale(struct window *window);
+
 void
 window_destroy(struct window *window);