Removed libpng from the list of hard dependencies.
Removed libpng from the list of hard dependencies.

It's just an option by now - like libjpeg and libXpm.

Signed-off-by: Tobias Stoeckmann <059b8b880f8441509ec8a65b50b4c6ae74ebea76@stoeckmann.org>

stoeckmann/xwallpaper,stoeckmann/xwallpaper