Support shadow catcher visibility

[wsxiaoys]

Is your feature request related to a problem? Please describe.

One common use case for rendered image, is to blend with an existing scene. It'll be super helpful if the plane is invisible while shadow presents.

For more information, see https://docs.blender.org/manual/en/latest/render/cycles/object_settings/object_data.html

Describe the solution you'd like

A shadowCatcher mask for material will be ideal.

[gkjohnson]

It sounds like this would be similar to the three.js ShadowMaterial, as well?

Would require adding another material field - this can be automatically determined by material if ShadowMaterial is found.