a407186dbf
Parallel shader compilation introduced `GPU_shader_cache_dir_clear_old`. The implementation was specific to OpenGL and could not be overwritten by other backends. This PR improves the implementation so the backend can have its own implementation. This is needed for upcoming changes to the Vulkan backend where we want to use similar mechanisms to speed up shader compilation and caching. Pull Request: https://projects.blender.org/blender/blender/pulls/127680