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.

source/blender/gpu/intern/gpu_shader_shared_utils.h

@@ -27,11 +27,10 @@
  * Some preprocessing is done by the GPU back-end to make it GLSL compatible.
  *
  * IMPORTANT:
- * - Don't add trailing comma at the end of the enum. Our custom pre-processor will now trim it
- *   for GLSL.
  * - Always use `u` suffix for enum values. GLSL do not support implicit cast.
  * - Define all values. This is in order to simplify custom pre-processor code.
- * - Always use uint32_t as underlying type.
+ * - (C++ only) Always use `uint32_t` as underlying type (`enum eMyEnum : uint32_t`).
+ * - (C only) do NOT use the enum type inside UBO/SSBO structs and use `uint` instead.
  * - Use float suffix by default for float literals to avoid double promotion in C++.
  * - Pack one float or int after a vec3/ivec3 to fulfill alignment rules.
  *