This changes the sampling routine to use the method described in
"A Simpler and Exact Sampling Routine for the GGXDistribution of Visible Normals"
by Eric Heitz.
http://jcgt.org/published/0007/04/01/slides.pdf
This avoids generating bad rays and thus improve noise level in screen-
space reflections / refraction.
This modifies the principled BSDF and the Glass BSDF which now
have better fit to multiscatter GGX.
Code to generate the LUT have been updated and can run at runtime.
The refraction LUT has been changed to have the critical angle always
centered around one pixel so that interpolation can be mitigated.
Offline LUT data will be updated in another commit
This simplify the BTDF retreival removing the manual clean cut at
low roughness. This maximize the precision of the LUT by scalling
the sides by the critical angle.
I also touched the ior > 1.0 approximation to be smoother.
Also incluse some cleanup of bsdf_sampling.glsl
- add the use of DRWShaderLibrary to EEVEE's glsl codebase to reduce code
complexity and duplication.
- split bsdf_common_lib.glsl into multiple sub library which are now shared
with other engines.
- the surface shader code is now more organised and have its own files.
- change default world to use a material nodetree and make lookdev shader
more clear.
Reviewed By: jbakker
Differential Revision: https://developer.blender.org/D8306
For now only compute GGX convolution. The GGX LUT used for the split sum approximation (UE4) is merged with the LTX mag LUT that uses the same parameters (theta and roughness)
