Material in core

[MathiasPaulin]

Pull Request Desription

This is a very early, and very incomplete) proposal to bring material data into Radium Core and improve interface for CPU usage.
Works right now with very limited functionalities.

TODO

• improve class Core::Tex to manage texture, or rely on openimageio
• declare (and implement) bsdf specific interface in material model :
  • bsdf (eval), taking w_i, w_o, N, uv as parameter and return the bsdf value for this configuration
  • sample : taking w_o, N, uv as parameter and return (w_i, pdf) of the generated direction
  • pdf , taking w_i, w_o, N, uv as parameter and return the probability of w_i wrt w_o, N, uv

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