Shadow catcher

[georgy95]

I've asked about the shadow plane in this thread, to which the recommendation was to write a custom:

• Integrator
• BSDF

It's actually a very difficult and important feature to get working correct and took Blender many many years to add properly. Now, it seems to tick all the boxes. Here is the full discussion that led implementing the shadow catcher.

P.S. I'm also willing to contribute a $250 amazon voucher for a PR that is merged as an incentive. Feel free to add more to this bounty. It doesn't have to be a full-featured PR, but at least a solid foundation that could be built on top of.