Public domain image and OGG Vorbis loading

Well, another change to SIEGE, this time without screenshots — namely, SIEGE has now received public domain image (via stb_image) and OGG Vorbis (via stb_vorbis) loading!

All the example programs have been modified to use this instead now, to be able to use SIEGE with image, audio (and, from before, font) loading without any external libraries!

Posted in SIEGE.