SDL_image: update libwebp to latest commit in 1.0.3-SDL branch

https://github.com/libsdl-org/SDL_image/commit/3f6e741918894a85dcdd884e1dfa388cec8225e5

From 3f6e741918894a85dcdd884e1dfa388cec8225e5 Mon Sep 17 00:00:00 2001
From: sezero <[EMAIL REDACTED]>
Date: Thu, 7 Apr 2022 22:37:56 +0300
Subject: [PATCH] update libwebp to latest commit in 1.0.3-SDL branch

---
 external/libwebp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/external/libwebp b/external/libwebp
index 0798d81..d13b8db 160000
--- a/external/libwebp
+++ b/external/libwebp
@@ -1 +1 @@
-Subproject commit 0798d81585fae3d5eb98e15d8754a4fb16857621
+Subproject commit d13b8dbe45a01df15c3896203f7734abc35230c8