Print warning messages for multipass blur

Multipass blur is not properly implemented for xrender backend. There is
visible artifacts when it is used. And it is quite difficult to
implement correctly and efficiently for the xrender backend.

Print a warning message for multipass blur so we can survey whether it
is actually been used.

No functionality is removed in this commit.

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

src/config.c

@@ -196,6 +196,12 @@ parse_conv_kern_lst(session_t *ps, const char *src, xcb_render_fixed_t **dest, i
       return false;
   }
 
+  if (i > 1) {
+    printf_errf("(): You are seeing this message because your are using multipass\n"
+        "blur. Please report an issue to us so we know multipass blur is actually been used.\n"
+        "Otherwise it might be removed in future releases");
+  }
+
   if (*pc) {
     printf_errf("(): Too many blur kernels!");
     return false;