cairo-util: Draw solid titlebar for frames with only buttons

Previously geometry was changed to leave space for titlebar if a frame
has only buttons but no title. This patch fixes theme_render_frame to
avoid transparent titlebar.

Signed-off-by: Boyan Ding <stu_dby@126.com>
diff --git a/shared/frame.c b/shared/frame.c
index 53f3f5f..5ea0e12 100644
--- a/shared/frame.c
+++ b/shared/frame.c
@@ -853,7 +853,7 @@
 
 	cairo_save(cr);
 	theme_render_frame(frame->theme, cr, frame->width, frame->height,
-			   frame->title, flags);
+			   frame->title, &frame->buttons, flags);
 	cairo_restore(cr);
 
 	wl_list_for_each(button, &frame->buttons, link)