Poll for Set Strength Node button
Full World settings support

Including solo, strength, color, MIS and MIS settings.

Strength and Color values are figured out per draw, looping through the chain of nodes connected from the World Output. The color node is chosen by the last node in the chain, or if the node only has Vector inputs and no others (such as Enviro Tex connected to Mapping, Enviro Tex is chosen). Strength node is chosen by the first node that has unconnected Value sockets (such as where a Math node is used, and one input is used to control the strength with the other connected to an image)