Make make_shadow() public

Some the refactored backends can use it.

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

src/render.c

@@ -443,7 +443,7 @@ static void paint_root(session_t *ps, const region_t *reg_paint) {
 	             ps->root_tile_paint.pict);
 }
 
-static xcb_image_t *make_shadow(session_t *ps, double opacity, int width, int height) {
+xcb_image_t *make_shadow(session_t *ps, double opacity, int width, int height) {
 	xcb_image_t *ximage;
 	int ylimit, xlimit;
 	int swidth = width + ps->cgsize;