v0.1.5-unstable

What's Changed

• Revamped path tracing integrator with support for mediums
• Added support for boxes
• Pixel Lambertian material now includes transparency textures
• Introduced mixture and colored isotropic mediums
• Added CSR support to edit sky gradient
• Replaced old dielectric, metal, and Lambertian models with improved versions:
  Better Dielectrics (Cook-Torrance)
  Better Metals (Cook-Torrance)
  Better Diffuse (Oren-Nayar)
  Layered materials