Make make_shadow() public

Some the refactored backends can use it.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/render.h

@@ -3,6 +3,7 @@
 #pragma once
 
 #include <xcb/render.h>
+#include <xcb/xcb_image.h>
 #include "region.h"
 
 typedef struct _glx_texture glx_texture_t;
@@ -34,3 +35,5 @@ void free_root_tile(session_t *ps);
 
 bool init_render(session_t *ps);
 void deinit_render(session_t *ps);
+
+xcb_image_t *make_shadow(session_t *ps, double opacity, int width, int height);