SDL_image: upgrade vendored libpng to v1.6.55 (a8b22)

From a8b22cb6bdb583d6fdb9495ad9a4cac2674d332a Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Tue, 17 Feb 2026 10:33:32 +0300
Subject: [PATCH] upgrade vendored libpng to v1.6.55

(cherry picked from commit df7f831cbebbc06252f1ee27b05a88a2514e6430)
---
 .gitmodules     | 2 +-
 external/libpng | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.gitmodules b/.gitmodules
index 69c36857..8d820f07 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -5,7 +5,7 @@
 [submodule "external/libpng"]
 	path = external/libpng
 	url = https://github.com/libsdl-org/libpng.git
-	branch = v1.6.53-SDL
+	branch = v1.6.55-SDL
 [submodule "external/libwebp"]
 	path = external/libwebp
 	url = https://github.com/libsdl-org/libwebp.git
diff --git a/external/libpng b/external/libpng
index ddf24cd7..1541b942 160000
--- a/external/libpng
+++ b/external/libpng
@@ -1 +1 @@
-Subproject commit ddf24cd74f8213e7a057c23e8c5c2135e1d0fad9
+Subproject commit 1541b942a25a17840617df941f2bc2905981cf9e