Part of #136993.
Share as much of the ShaderCompiler implementations as possible.
Remove the ShaderCompiler/ShaderCompilerGeneric split and make most of
its functions non virtual.
Move the `get_compiler` function from `Context` to `GPUBackend` and
creation/deletion to `GPUBackend::init/delete_resources`.
Add a `batch_cancel` function to `ShaderCompiler` (needed for the
GPUPass refactor).
As a nice extra, the multithreaded OpenGL compilation has become faster
too.
The barbershop materials + EEVEE static shaders have gone from 27s to
22s.
I have not observed any performance difference on Vulkan or Metal.
Pull Request: https://projects.blender.org/blender/blender/pulls/136676
992e7c95a7