SDL_image: upgrade vendored libpng to 1.6.51

From eaf87a8ca32b0ec8ad5c42d71d48f74671fd0f1e Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Sat, 29 Nov 2025 20:37:10 +0300
Subject: [PATCH] upgrade vendored libpng to 1.6.51

---
 .gitmodules     | 2 +-
 external/libpng | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.gitmodules b/.gitmodules
index 3bf0d537..56f7f1d7 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -5,7 +5,7 @@
 [submodule "external/libpng"]
 	path = external/libpng
 	url = https://github.com/libsdl-org/libpng.git
-	branch = v1.6.50-SDL
+	branch = v1.6.51-SDL
 [submodule "external/libwebp"]
 	path = external/libwebp
 	url = https://github.com/libsdl-org/libwebp.git
diff --git a/external/libpng b/external/libpng
index 8c90f8f2..efecf112 160000
--- a/external/libpng
+++ b/external/libpng
@@ -1 +1 @@
-Subproject commit 8c90f8f24b58e11eecd4a5b9bdbedf97f9ccfd64
+Subproject commit efecf1124f248dd325925bb7978f7f66f373541a