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.

[jstone-lucasfilm]

I'd probably recommend against changing this line in clang-format, as I've run into cases where it will have unintended side effects that affect compilation.

Here's one example from MaterialXFormat/File.cpp, where the current include order correctly places windows.h and unistd.h first in their respective blocks:

#if defined(_WIN32)
    #define WIN32_LEAN_AND_MEAN
    #include <windows.h>
    #include <direct.h>
#else
    #include <unistd.h>
    #include <sys/stat.h>
    #include <dirent.h>
#endif

But with SortIncludes set to true, this gets reformatted to the following:

#if defined(_WIN32)
    #define WIN32_LEAN_AND_MEAN
    #include <direct.h>
    #include <windows.h>
#else
    #include <dirent.h>
    #include <sys/stat.h>
    #include <unistd.h>
#endif

[ld-kerley]

I would still like to find a path forwards to being able to use clang-format. A lot of reviews get hung up on formatting issues that could be avoided if we could use the tools to enforce the formatting. As well as if we use a formatting tool, it makes rebasing really old branches a LOT easier.

for cases like this - we could easily wrap the section of includes - or any other small part of the code - we don't want auto-formatted in the comments that disable the formatter.

// clang-format off
<code that is not reformatted>
// clang-format on

This would theoretically give us the best of both worlds.