ported volume rendering
Merge branch 'feature/new_renderer' of github.com:vrsys/guacamole into feature/new_renderer
added make_pass to VolumePass
Add comments.
Start physically based shading implementation.
Load dds files.
PBR lighting.
Load Albedo texture.
Use metalness and roughness.
Add cerberus material.
Use tga textures.
Renders nice and shiny.
Add PBR pass.
Restore to original version.
Use PhysicallyBasedShadingPass.
Add keybindings to toggle between classic and new lighting.
Fix documentation.
Add brightness to spotlight.
Add make_pass.
Move callback code into separate functions.