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/backend.c

@@ -1,4 +1,5 @@
 #include "backend.h"
+#include "common.h"
 
 backend_info_t *backend_list[NUM_BKEND] = {[BKEND_XRENDER] = &xrender_backend};