Add rasterizer displacement support

[hybridherbst]

Summary

This PR adds hardware shader generation for displacement, including vertex-stage displacement emission, texture-driven displacement dependencies, compound displacement calls, multi-output displacement handling, and derivative-based normal recomputation for displaced surfaces.

This allows MaterialXView, MaterialXGraphEditor and others to work with, display, and render displacement materials.

This contribution is funded by Needle

[jstone-lucasfilm]

Thanks so much for this proposal, @hybridherbst! We should reserve some time to dive into the details of this work, but from a high level, it addresses a very important feature that we've long wanted to include in MaterialX.

[jstone-lucasfilm]

@hybridherbst Would you mind resolving the merge conflicts between this PR and recent commits from the community? That should help us to properly test and review the proposal.