[TODO] [Tydra] Support custom texture loader callback

[syoyo]

Currently, TinyUSDZ only supports loading texture asset with known fie format.

https://github.com/syoyo/tinyusdz/blob/caa6d877ff5448a3b1ca48497e0a07f374b5098c/src/tydra/render-data.cc#L2599

But we'd like to allow texture loader callback interface to load texture with unknown format.

[syoyo]

Oh Callback interface was already defined here: https://github.com/syoyo/tinyusdz/blob/ea113063ab06cddcf7597a09f9203b4ef1284d60/src/image-loader.hh#L31

So we can use it in Tydra's TextureLoader(T.B.D.)