Allow precompiling specialization constants variations in parallel.
Only supported in OpenGL as the rest of the batch compilation API,
on the other backends the function is a no-op.
This also moves the `SpecializationConstant` from
`gpu_shader_create_info` (private API) into`GPU_common_types`
(public API).
Pull Request: https://projects.blender.org/blender/blender/pulls/122796
22652b305e