libpng Fix, 21.0 branch (2021.02.18.)

Made Athena set up the runtime environment of LCG's png package explicitly. With no package in Athena explicitly depending on libpng, the project should set it up itself. As it is needed at runtime by ROOT, to be able to "interact" with PNG images.

This is to fix the issue pointed out by @rustem in: https://groups.cern.ch/group/hn-atlas-PATHelp/Lists/Archive/Flat.aspx?RootFolder=%2Fgroup%2Fhn%2Datlas%2DPATHelp%2FLists%2FArchive%2FProblem%20with%20creating%20TCanvas%20using%20ROOT%20shipped%20with%20Athena21%2E0%2E121&FolderCTID=0x0120020084D7E80CB0C3394A8D54EF07C309B044

I was kind of hoping that somebody else would make this update, but c'est la vie...