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.h

@@ -1,6 +1,6 @@
 #pragma once
-#include <xcb/xcb_image.h>
 #include <xcb/render.h>
+#include <xcb/xcb_image.h>
 
 #include <stdbool.h>
 
@@ -8,6 +8,7 @@
 
 typedef struct session session_t;
 typedef struct win win;
+typedef struct conv conv;
 
 bool build_shadow(session_t *ps, double opacity, const int width, const int height,
                   xcb_render_picture_t shadow_pixel, xcb_pixmap_t *pixmap,
@@ -17,3 +18,6 @@ xcb_render_picture_t
 solid_picture(session_t *ps, bool argb, double a, double r, double g, double b);
 
 void paint_all_new(session_t *ps, region_t *region, win *const t);
+
+xcb_image_t *make_shadow(xcb_connection_t *c, const conv *kernel,
+                         const double *shadow_sum, double opacity, int width, int height);