[Bf-cycles] Volume Refraction shader

[Mircea Kitsune]

This is something I've been wanting to get to the developers to for a 
while. It's a feature I hoped to use on several occasions, but 
unfortunately it doesn't exist nor seems to be in any official plans as 
of yet. Please consider a solution if possible. Thank you.

Suggestion request: Implementing a volume refraction shader. Either by 
allowing the existing Refraction BSDF to work on the Volume material 
output, or by defining a new Volume Refraction shader if that is a 
better approach.

Suggested functionality: When a ray passes through a volume affected by 
this shader, its trajectory is bent, just like Glass BSDF or Refraction 
BSDF does for surfaces. Optionally, an IOR value may let users define 
the probability and / or angle at which rays have their course modified, 
whereas a Roughness value adjusts how blurry the view gets through such 
a volume... the two ideally operate in an independent way so that 
refraction and blur can work without one another.

Purpose 1: The most important practical use of this shader is proper 
heat haze, emulating the air deformations we see above hot ovens / jet 
engines / roads during summer days. As a workaround, this is already 
possible to do with render nodes, by rendering your volume as a black & 
white mask on a different render pass then plugging it into the Vector 
input of a Displace node through which the main render is passed. 
However this is just a workaround that so happens to work: It leads to 
quality loss due to smudging and stretching the render result, causes 
empty spaces if the transformation of the mask warps near an edge, is 
much less realistic compared to the ray actually traveling a different 
trajectory, and might ultimately be slower depending on what 
calculations are implied.

Purpose 2: This would allow a greater level of detail for objects such 
as crystals, diamonds, gems, ice crystals, and other types of rough 
glass. You can already design realistic crystals in Cycles, by 
customizing the IOR and Roughness of a Glass BSDF or Refraction BSDF, 
causing different spots to refract or blur in different amounts. However 
you can't differentiate such spots volumetrically inside the crystal 
itself, and can't create rough blurry strands or inner glass pieces that 
have a different refraction angle from others. This can of course be 
worked around by adding smaller meshes inside the main crystal and 
giving them different materials, but this technique will cause sharp 
edges and is less correct and flexible compared to volumes in many cases.