Expose field values as outputs

[t3kt]

It would be useful for post-processing to be able get an output buffer that has the results of a field evaluated at the surface hit point (if any) for each pixel.
Could use it for things like fields that sweep through space

[t3kt]

Maybe have an exposeValue operator, which runs its input op and stores a copy of the result in either:

• its own global variable, and shaderBuilder generates code to copy from those variables into output buffers.
• or the op could just write directly to the output buffer.

It will be important to include render stage filtering, since values coming from the primary ray cast could be overwritten by things like shadow ray casts.

Could also be useful to have the ability to have it only store the result from an SDF input if the result represents a surface hit. Though perhaps that would end up being effectively done anyway since that would be the last step of the ray.

[t3kt]

Need to either support arbitrary output buffers added by ops, or add a fixed set of them which ops can choose from (customOut1, etc). Arbitrary buffers will involve a fair amount of changes to shaderBuilder infrastructure, so maybe avoid that.