Observable().subscribe(...).future() aka DeferredFuture should forward progress on chained pipeline

[vpicaver]

For example this pipeline should work:

Run 100 concurrent jobs, then setup another 100 jobs, finally setup another 100 jobs.

The user should be able to get the correct progress by watching the last future in this pipeline. Below is a test-case.

    QVector<int> ints(100);
    std::iota(ints.begin(), ints.end(), ints.size());
    std::function<int (int)> func = [](int x)->int {
        QThread::msleep(100);
        return x * x;
    };
    QFuture<int> mappedFuture = QtConcurrent::mapped(ints, func);

    auto nextFuture = AsyncFuture::observe(mappedFuture).subscribe([ints, func](){
        QFuture<int> mappedFuture2 = QtConcurrent::mapped(ints, func);
        return mappedFuture2;
    }).future();

    bool nextExecuted2 = false;
    auto nextFuture2 = AsyncFuture::observe(nextFuture).subscribe([&nextExecuted2, ints, func](){
        QFuture<int> mappedFuture2 = QtConcurrent::mapped(ints, func);
        nextExecuted2 = true;
        return mappedFuture2;
    }).future();

    int progress = -1;
    AsyncFuture::observe(nextFuture2).onProgress([&progress, nextFuture2](){
        QVERIFY2(progress <= nextFuture2.progressValue(), QString("%1 <= %2").arg(progress).arg(nextFuture2.progressValue()).toLocal8Bit());
        progress = nextFuture2.progressValue();
    });

    await(nextFuture2);

    QCOMPARE(nextFuture2.progressMinimum(), 0);
    QCOMPARE(nextFuture2.progressMaximum(), ints.size() * 3);

    QCOMPARE(nextExecuted2, true);
    QCOMPARE(nextFuture2.progressValue(), ints.size() * 3);

[vpicaver]

This issue has been fixed in my fork.