This allows multiple threads to request different specializations without
locking usage of all specialized shaders program when a new specialization
is being compiled.
The specialization constants are bundled in a structure that is being
passed to the `Shader::bind()` method. The structure is owned by the
calling thread and only used by the `Shader::bind()`.
Only querying for the specialized shader (Map lookup) is locking the shader
usage.
The variant compilation is now also locking and ensured that
multiple thread trying to compile the same variant will never result
in race condition.
Note that this removes the `is_dirty` optimization. This can be added
back if this becomes a bottleneck in the future. Otherwise, the
performance impact is not noticeable.
Pull Request: https://projects.blender.org/blender/blender/pulls/136991
