render: Expose render_overlay

This function updates the overlay that foot uses. It will be used to
update the overlay when the flash effect ends.

render.h

@@ -31,6 +31,8 @@ bool render_xcursor_set(
     struct seat *seat, struct terminal *term, enum cursor_shape shape);
 bool render_xcursor_is_valid(const struct seat *seat, const char *cursor);
 
+void render_overlay(struct terminal *term);
+
 struct render_worker_context {
     int my_id;
     struct terminal *term;