Commenting of #include directives is not respected by shader generation

[ld-kerley]

While helping Masuo get the hex tiling PR working in MSL, I spent a long time being confused....

Commenting out #include lines does not stop them from being included.

//#include "mx_math.glsl"

The mx_math.glsl file was still being included, causing compile errors.

In this case we don't actually need to include this file, but in general we need shader generation to be robust, as its a bit of a blackbox for shader developers.

[jstone-lucasfilm]

Good catch, @ld-kerley and @msuzuki-nvidia, and I believe that logic is currently implemented here in the shader generation system:

MaterialX/source/MaterialXGenShader/ShaderStage.cpp

Line 315 in ed450e8

const string& INCLUDE = _syntax->getIncludeStatement();