[TODO] [Tydra] Support custom texture loader callback

[Syoyo Fujita]

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

tinyusdz/src/tydra/render-data.cc

Line 2599 in caa6d87

 auto result = tinyusdz::image::LoadImageFromFile(resolvedPath); 

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

[Syoyo Fujita]

Oh Callback interface was already defined here:

tinyusdz/src/image-loader.hh

Line 31 in ea11306

 /// User-defined Image asset loader 

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