Adding thick borders to the buildable Supply Centers to the Ancient Mediterranean variant via Indexed PNGs

[Moxanity]

My previous pull request (#687) didn't use Indexed PNGs resulting in errors. This one is using Indexed PNGs, exported using GIMP.

[kestasjk]

Thanks for offering this Moxanity. Is this variant on vDip?

[Moxanity]

Yes, it is. Why? https://www.vdiplomacy.com/variants.php?variantID=9