Impossible to provide custom thread pool provider

I tried to add a custom thread pool provider for OpenEXR (using the internal `DefaultThreadPoolProvider` as reference) via `IlmThread::ThreadPool::globalThreadPool().setThreadProvider()`. It seems that the provider should call `task->group()->_data->removeTask();` after executing the task. However, the definition of `IlmThread::TaskGroup::Data` is not provided, making it impossible to do this.

Is one expected to be able to customize OpenEXR like this by the application? If so, then I think some changes are needed in order to allow this.

For reference, here's the custom thread pool provider that I tried to use but it (expectedly) fails to compile on MSVC with:
``` error C2027: use of undefined type 'IlmThread_3_1::TaskGroup::Data'```

Complete source:
```
class TBBThreadProvider : public IlmThread::ThreadPoolProvider
{
public:
TBBThreadProvider() = default;
~TBBThreadProvider() noexcept override { finish(); }

int numThreads() const override { return myArena->max_concurrency(); }

void setNumThreads(int count) override
{
finish(); // TODO: Is this strictly needed?
myArena = std::make_unique(count);
}

void addTask(IlmThread::Task *task) override
{
myArena->execute([&]
{
myGroup.run([&]
{
task->execute();
task->group()->_data->removeTask();
delete task;
});
});
}

void finish () override
{
myArena->execute([&] { myGroup.wait(); });
}

private:
std::unique_ptr myArena = UTmakeUnique();
tbb::task_group myGroup;
};
```

Start with IlmThread::ThreadPool::globalThreadPool(), ThreadPoolProvider, TaskGroup::Data, and the internal DefaultThreadPoolProvider used as the reference. Determine whether application-supplied providers are supported and what public task-completion API is required; done means the intended customization path is defined and the supplied provider can compile and manage task completion on MSVC.

Written by the indexing model from the issue text.