Add matrix type switch nodes

[ld-kerley]

• ND_switch_matrix33
• ND_switch_matrix44
• ND_switch_matrix33I
• ND_switch_matrix44I

[jstone-lucasfilm]

This looks great to me, thanks @ld-kerley, and I'm CC'ing @krohmerNV and @dbsmythe for their thoughts on the MDL implementation and specification revisions.

[krohmerNV]

@jstone-lucasfilm this will probably not compile with MDL 1.6 and 1.7 because the stdlib_1_6.mdl needs the definitions too (or it should be there only).. anyhow I'll fix this as soon as I can.

But a general question. Isn't it possible to implement the switch in mtlx directly using a graph? this hard-coded number of options doesn't look nice.

[jstone-lucasfilm]

@krohmerNV Thanks for the catch, and it's a good reminder that we still want to integrate MDL shader validation into our GitHub Actions tests.

If there's an efficient graph-based implementation of switch, we'd definitely be open to that improvement, and it would reduce the dependence on language-specific implementations.

[ld-kerley]

@krohmerNV - are the additions you're suggesting are needed to stdlib_1_6.mdl in this PR already - here ?

Or are other changes needed?

[krohmerNV]

@ld-kerley, you are right. That's what I meant. Sorry.
I'll check locally to verify and let you know.