NME custom blocks

[Blake]

Here’s the new thread for the second draft proposal/PR for NME custom blocks, based on json files

NME custom blocks take two

Hey y’all, here’s the first draft of the second incarnation of custom blocks for NME! It’s based on user-defined, simple json files. So far only the features needed to port MultiplyBlock have been implemented. Saving and loading materials that use custom blocks seems to be working well also. The next steps I think are to provide default values for all of the properties, which have been omitted mainly to facilitate testing, and to port a more complicated block, like WorleyNoise3D…