Use functions in backend/backend_common.c

Build backend_common.c even when new_backend=false.

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

src/backend/backend_common.c

@@ -46,9 +46,8 @@ solid_picture(session_t *ps, bool argb, double a, double r, double g, double b)
 	return picture;
 }
 
-static xcb_image_t *
-make_shadow(xcb_connection_t *c, const conv *kernel, const double *shadow_sum,
-            double opacity, int width, int height) {
+xcb_image_t *make_shadow(xcb_connection_t *c, const conv *kernel,
+                         const double *shadow_sum, double opacity, int width, int height) {
 	/*
 	 * We classify shadows into 4 kinds of regions
 	 *    r = shadow radius