Add Scripts/packagePregenSpirvToolsHeaders script to automate packaging Spirv-Tools
headers in support of the fetchDependencies --skip-spirv-tools-build option.
Update Docs/Whats_New.md.
Refactor build scripts.
Update paths to MoltenVKGLSLToSPIRVConverter framework in demos.
Fix rare build race condition on MoltenVKShaderConverter link to MoltenVK.