Revert "main/libpng: upgrade to 1.6.24"

Seems like it broke ABI. It resulted in this error when starting firefox:

XPCOMGlueLoad error for file /usr/lib/firefox-46.0.1/libxul.so:
Error relocating /usr/lib/firefox-46.0.1/libxul.so: png_get_next_frame_x_offset: symbol not found
Couldn't load XPCOM.

This reverts commit 4de614d4.

main/libpng/APKBUILD

 # Contributor: Carlo Landmeter <clandmeter@gmail.com>
 # Maintainer: Natanael Copa <ncopa@alpinelinux.org>
 pkgname=libpng
-pkgver=1.6.24
-_apngver=1.6.23
-pkgrel=0
+pkgver=1.6.23
+_apngver=$pkgver
+pkgrel=2
 pkgdesc="Portable Network Graphics library"
 url="http://www.libpng.org/"
 arch="all"
@@ -12,7 +12,7 @@ depends=""
 depends_dev="zlib-dev"
 makedepends="$depends_dev gawk"
 subpackages="$pkgname-doc $pkgname-dev"
-source="http://downloads.sourceforge.net/$pkgname/$pkgname-$pkgver.tar.xz
+source="http://downloads.sourceforge.net/$pkgname/$pkgname-$pkgver.tar.gz
 	http://downloads.sourceforge.net/sourceforge/libpng-apng/libpng-$_apngver-apng.patch.gz
 
 	libpng-fix-arm-neon.patch
@@ -46,12 +46,12 @@ package() {
 		"$pkgdir"/usr/share/licenses/$pkgname/LICENSE || return 1
 }
 
-md5sums="ffcdbd549814787fa8010c372e35ff25  libpng-1.6.24.tar.xz
+md5sums="a49e4cc48d968c79def53d082809c9f2  libpng-1.6.23.tar.gz
 bf44929a1c272f585a11baeca05af500  libpng-1.6.23-apng.patch.gz
 e58c8d9a4086999456de13b2c4b9e224  libpng-fix-arm-neon.patch"
-sha256sums="7932dc9e5e45d55ece9d204e90196bbb5f2c82741ccb0f7e10d07d364a6fd6dd  libpng-1.6.24.tar.xz
+sha256sums="74a9db9ddd66caa64be82bfcb3cbec4fc4394ceb1053cb7e94b2a0aa9b78eb88  libpng-1.6.23.tar.gz
 08906e0639a953f6be2d47857661cbdf04dcab93d3bf9f8cb0f7675567b07ad3  libpng-1.6.23-apng.patch.gz
 db38cd0a8ae637e1bf2dca464a2068a40576346d16ae929f20dc712d21870316  libpng-fix-arm-neon.patch"
-sha512sums="7eccb90f530a9c728e280b2b1776304a808b5deea559632e7bcf4ea219c7cb5e453aa810215465304501127595000717d4b7c5b26a9f8e22e236ec04af53a90f  libpng-1.6.24.tar.xz
+sha512sums="f5cf137186fc0a10deb3ce54b1961cb9a358cec380cd3f8c21d2726e68348574c49a5a6808ecd79736737e970fa41ae83626811cb72b059bbf6054fabf6639f8  libpng-1.6.23.tar.gz
 105e7f70d1292f4a059e62fda7c4ffbfcd4eacd3c11cd1fbdd09bdcf2b1469a54ee864c4f5f22afe9e17ddfd1d10196661a69bb4eb469173a3bfc789181923ce  libpng-1.6.23-apng.patch.gz
 30c8389d075b90c0e30c0be8d7630cbf98c87c610f414c529485630dd0411cced28852d2fecc96bca72bc712f0f7e13358f9fc47b6d7b8c7806d34ebeef20514  libpng-fix-arm-neon.patch"