[feature] Shader code testing instruction and examples

# Shader code testing procedures that includes mobile devices

Some instructions or help on how to test shader code generated by naga-oil, or the wgsl code in the codebase in general. A lot of the functions in the shader files could really benefit from some way of testing them on real hardware. Since reading [this from Unity](https://docs.unity3d.com/es/530/Manual/SL-DataTypesAndPrecision.html) states the need to test shader code that is supposed to run on mobile devices, where the hardware capabilities and performance features can lead to numerical issues.

Many of the currently open issues regarding crashes, issues on mobile devices, can probably be traced to shadercode that behaves differently on mobile units.

The PC GPUs, according to the unity docs, behave much more coherently, but the code itself would very much benefit from some unit testing in anyway.