Update libpng to 1.6.51 (#1862)

Author: Rangi42

.github/scripts/build_libpng.sh

@@ -1,13 +1,13 @@
 #!/bin/bash
 set -euo pipefail
 
-pngver=1.6.50
+pngver=1.6.51
 
 ## Grab sources and check them
 
 curl -LOJ "http://prdownloads.sourceforge.net/libpng/libpng-$pngver.tar.xz?download"
 # Brew doesn't provide any sha256sum, so we're making do with `sha2` instead.
-if [ "$(sha2 -q -256 libpng-$pngver.tar.xz)" != 4df396518620a7aa3651443e87d1b2862e4e88cad135a8b93423e01706232307 ]; then
+if [ "$(sha2 -q -256 libpng-$pngver.tar.xz)" != a050a892d3b4a7bb010c3a95c7301e49656d72a64f1fc709a90b8aded192bed2 ]; then
 	sha2 -256 libpng-$pngver.tar.xz
 	echo Checksum mismatch! Aborting. >&2
 	exit 1

.github/scripts/get_win_deps.ps1

@@ -16,8 +16,8 @@ function getlibrary ([string] $URI, [string] $filename, [string] $hash, [string]
 }
 
 getlibrary 'https://www.zlib.net/zlib131.zip' 'zlib.zip' '72af66d44fcc14c22013b46b814d5d2514673dda3d115e64b690c1ad636e7b17' .
-getlibrary 'https://github.com/pnggroup/libpng/archive/refs/tags/v1.6.50.zip' 'libpng.zip' 'f6bb2544d2cf5465af3a695dee0b7eacff82f11a50aa4672ef0e19df6e16d455' .
+getlibrary 'https://github.com/pnggroup/libpng/archive/refs/tags/v1.6.51.zip' 'libpng.zip' 'd1e8544445152f5c71fc58d06067da80f3f642f268d2c3e1114a7f34fd739e32' .
 getlibrary 'https://github.com/lexxmark/winflexbison/releases/download/v2.5.25/win_flex_bison-2.5.25.zip' 'winflexbison.zip' '8d324b62be33604b2c45ad1dd34ab93d722534448f55a16ca7292de32b6ac135' install_dir
 
 Move-Item zlib-1.3.1 zlib
-Move-Item libpng-1.6.50 libpng
+Move-Item libpng-1.6.51 libpng

.github/scripts/mingw-w64-libpng-dev.sh

@@ -1,13 +1,13 @@
 #!/bin/bash
 set -euo pipefail
 
-pngver=1.6.50
+pngver=1.6.51
 arch="$1"
 
 ## Grab sources and check them
 
 wget http://downloads.sourceforge.net/project/libpng/libpng16/$pngver/libpng-$pngver.tar.xz
-echo 4df396518620a7aa3651443e87d1b2862e4e88cad135a8b93423e01706232307 libpng-$pngver.tar.xz | sha256sum -c -
+echo a050a892d3b4a7bb010c3a95c7301e49656d72a64f1fc709a90b8aded192bed2 libpng-$pngver.tar.xz | sha256sum -c -
 
 ## Extract sources and patch them