Parsing shaders

I've done some more work on the Maya translator to support the new concept of shaders. 

A "Shader" in Aurora is responsible for prelighting only. They're somewhat atomic pieces of logic that given shading geometry and some parameters produce either a color or float (for now. I'll introduce manifolds, prim vars and other fun concepts later) which forms a virtual node graph that eventually feeds into the material properties of the "Material". The material interfaces with the integrators, and is responsible for handing over a singe BxDF during light transport. So far I've only written a texture2d shader with exr support and a noise3d shader with support for various transforms and spaces, but now that I have a generalized framework for shading they're pretty straight forward/quick to write, so I'll add a bunch more as I get time.

The Python and Maya material interface is monolithic/ubershader-style for now - I just piggy back on the existing Maya material node, and throw a bunch of my own attributes on there and parse these during scene translation. Internally it's all very modular, but while I figure out how to best parse/manage that elegantly it'll do for now. I suspect I can leverage the Hypershade in Maya, but I'll need to flesh out how to best handle it while parsing and also how to write custom nodes in Maya. Testing it out on a bigger scene it's behaving fine, but it's a pretty big impact on render times. It's nice to be able to control things interactively in Maya, though (although I haven't implemented any IPR style rendering yet rendering with only a single bounce is pretty fast).

The framework puts a clean line between prelighting and lighting allowing me to adaptively cache the former and stay unbiased. There's no caching mechanism implemented yet, though, so shaders are being re executed upwards of thousands of times more than needed.

A quick test on a medium sized scene:
1.5 mill polys, 9 light sources, multiple different materials, some with a procedural noise shader. 2048x1024 res, 8k samples per pixel and 16 light bounces. Render time was around 12h on my laptop, but it should drop considerably once I'm done with the shader cache. The lighting and material settings are pretty arbitrary, but I'm thinking I'll try and use this scene as a testing ground for my engine going forward by polishing it up a bit with textures and some proper lookdev and lighting. (Although the thought of UV mapping this beast isn't exactly intriguing..)

-Espen