Add make_shadow and paint_all_new to backend_common.c

Also made make_shadow private in render.c

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

src/backend/xrender.c

@@ -1,5 +1,6 @@
 #include <assert.h>
 #include <math.h>
+#include "common.h"
 #include "backend/backend.h"
 #include "backend_common.h"
 #include "utils.h"