[BIRP] Support convoluted planar reflections

[johnrossporter]

Please describe the desired feature/behaviour.

In the built-in PBR rendering, reflections from reflection probes are convoluted, where they appear progressively blurrier down the mip chain and are rendered as such depending on the roughness value of the material. The Crest shader has a roughness input, which works correctly with reflection probes, but because planar reflection has no convolution step, it does not work correctly with it. I propose bringing this functionality to Crest.

Supporting images/mockups, or example reference videos are very helpful to communicate ideas.

There is an existing implementation of planar reflection from the Adam demo that integrates planar reflection with a convolution step- https://github.com/keijiro/AdamPlaneReflection