Add Windows testing

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

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+#!/bin/sh
+
+# This script was written by ISSOtm while looking at Arch Linux's PKGBUILD for
+# the corresponding package. (And its dependencies)
+# https://aur.archlinux.org/packages/mingw-w64-libpng/
+
+set -e
+
+pngver=1.6.37
+_apngver=$pngver
+_arch="$1"
+
+
+## Install mingw-configure and mingw-env (both build dependencies)
+
+install -m 755 .github/actions/mingw-env.sh /usr/bin/mingw-env
+
+sed "s|@TRIPLE@|${_arch}|g" .github/actions/mingw-configure.sh > ${_arch}-configure
+install -m 755 ${_arch}-configure /usr/bin/
+
+
+## Grab sources and check them
+
+wget http://downloads.sourceforge.net/sourceforge/libpng/libpng-$pngver.tar.xz
+wget http://downloads.sourceforge.net/project/apng/libpng/libpng16/libpng-$_apngver-apng.patch.gz
+sha256sum -c .github/actions/mingw-w64-libpng-dev.sha256sums
+
+## Extract sources
+
+tar -xf libpng-$pngver.tar.xz
+gunzip libpng-$_apngver-apng.patch.gz
+
+
+## Start building!
+
+cd libpng-$pngver
+# Patch in apng support
+patch -p0 ../libpng-$_apngver-apng.patch
+
+mkdir -p build-${_arch}
+cd build-${_arch}
+${_arch}-configure LDFLAGS=-static-libgcc
+make
+make install