Replacing CONFIG_VNC_PNG with CONFIG_PNG

Libpng is only detected if VNC is enabled currently. This patch adds a
generalised png option in the meson build which is aimed to replace use of
CONFIG_VNC_PNG with CONFIG_PNG.

Signed-off-by: Kshitij Suri <kshitij.suri@nutanix.com>

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220408071336.99839-2-kshitij.suri@nutanix.com>

[ kraxel: add meson-buildoptions.sh updates ]
[ kraxel: fix centos8 testcase ]
[ kraxel: update --enable-vnc-png too ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

--enable-vnc-png fixup

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>