Added TEXCOORD_1 support

glTF sometimes authors a second TEXCOORD set (first one is TEXCOORD_0, 0 indexed numbering). The official spec only requires support for the first two TEXCOORD sets, even though there can theoretically be any number of them in a model. That said, major model exporters don’t author any more than the two sets, so to avoid extra work, I only added support for 0 and 1.
(not merged into main yet, I want to polish it a little and maybe add KHR_texture_transform support on top of it first)