Separate Shader Generation Exception header

[ld-kerley]

Create separate header for the shader generation exception. This reduces the dependency on the shader generator header.

Note - the exception header is still included in the main shader generation header for backwards compatibility

[jstone-lucasfilm]

This looks like a good proposal to me, thanks @ld-kerley, and I had just minor notes about maintaining existing conventions for include order.

[ld-kerley]

I think changing .clang-format from

SortIncludes: false

to

SortIncludes: CaseSensitive
IncludeBlocks: Preserve

would perform the sorting that I think you're outlining.

I also think it would be really helpful if these coding conventions were either enforced rigorously by clang-format or that they were documented somewhere.