Move the `StaticShader` class from Workbench to `GPU_shader` and make
compilation thread-safe (Shader usage is still not thread-safe).
Use `StaticShader`s for all shader caches.
Subdivision shaders are still not ported.
(Part of #134690)
Pull Request: https://projects.blender.org/blender/blender/pulls/134812
f930d71a1e