Waveform source: support really slow rates with fractional steps

[guruofquality]

When we dont want to or cant generate wave tables any larger, and approximation of the waveform could be made by accumulating fractional steps and rounding off the index to the nearest table entry:

if (stepFloat < 1.0)
{
        for (size_t i = 0; i < outPort->elements(); i++)
        {
            out[i] = _table[_index & _mask];
            _offset += _stepFloat;
            //TODO when abs(offset) >= 1.0, move its integer part into index
        }
}