Refractive materials

[Duttenheim]

Add support for refractive materials. This would require a refractive material shader which reads from the forward-lit render target. It should provide the following support:

• Refract opaque materials, particles and self-lit (emissive materials)
• Rough refractions, select downsampled mip of lighting buffer to sample from based on roughness
• Index of refraction and scale based on transform https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_volume (No need to follow the glTF specification for parametrization)