Add createShaderMaterial to GPUComputationRenderer

[mattrossman]

Why

See the JS definition here: https://github.com/mrdoob/three.js/blob/r133/examples/jsm/misc/GPUComputationRenderer.js#L302-L318

This function's usage is also mentioned in the comments at the top of the file: https://github.com/mrdoob/three.js/blob/r133/examples/jsm/misc/GPUComputationRenderer.js#L84-L85

What

Adds the missing createShaderMaterial function definition to GPUComputationRenderer.

I didn't see an existing GPGPU test and I'm reluctant to write my own since there's a lot of moving parts and shader fluff in the three.js examples for those, seems like it'd be a lot of work to properly cover all the parts of GPUComputationRenderer. If you have a suggestion for a simple test case then I can add it though.

Checklist

• Ready to be merged

[joshuaellis]

I've added a test and removed what I believe to be a weakness in our type system regards to RenderTarget a generic empty object. The test highlighted this for me which is nice.