See #129009 for context.
The preprocessor parses metadata and writes a header file containing
an inline function that inits the `GPUSource` with the metadata.
These header files are then included inside `gpu_shader_dependency.cc`.
This still keep the usage of the `metadata` enums and classes to avoid
pulling the whole blender module inside the preprocessor executable.
This speeds-up startup time in Debug build:
`gpu_shader_dependency_init`
- Before : 37ms
- After : 4ms
I didn't measure release, but it is unlikely to be noticeable (in the
order of 4ms > 1ms).
Pull Request: https://projects.blender.org/blender/blender/pulls/138070
d24b0e4a4e