GPUShader: Add GLSL source modification pass to support enums

This uses a light parser / string modification pass to convert
C++ enum declaration syntax to GLSL compatible one.

GLSL having no support for enums, we are forced to convert the
enum values to a series of constant uints.

The parser (not really one by the way), being stupidly simple,
will not change anything to the values and thus make some C++
syntax (like omitting the values) not work.

The string replacement happens on all GLSL files on startup.
I did not measure significant changes in blender startup speed.
There is plans to do all of this at compile time.

We limit the scope of the search to `.h` and `.hh` files to prevent
confusing syntax in `.glsl` files.

There is basic error reporting with file, line and char logging
for easy debuggabiliy.

The requirements to use this enum sharing system are already listed in
`gpu_shader_shared_utils.h` and repeated on top of the preprocessor
function.

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