glTF exports don't export double-sided materials as double-sided/blendmodes

[Marlamin]

Requires a light rework of how we export textures as we currently do it the opposite way it should probably be done as we go straight from textures to glTF while we should probably go from skin -> texUnit -> textures/materials, the latter where we can pick up on the two-sided flags.

Same goes for blendmodes.