TD renderer discontinuities for headtracking

With fast changes in the headtracking, in combination with low-frequency tones, the TD renderer can have discontinuities at the subframe boundaries. This is seen in #1195, but has also been observed in some other test cases.

The probable cause is that the filter interpolation is only triggered for the metadata updates every 20 ms, but not at the subframe boundaries. The filter interpolation should be activated for all subframes. When doing this, the update frame of the metadata should remain the same.

I tested the fix on what should be the worst case, 48 kHz 7.1 with headtracking. The impact is 331.783 - 331.199 = 0.584 WMOPS increase. This seems like an acceptable increase for the quality improvement for this type of signal.