DPLII Upmix to 5.1

[spoRv]

AFAIK, these matrix coefficients are the right ones.

And never read about something completely different like the ones found here: 🔍https://dvedaa.github.io/posts/gsoc-2020/

The main problem is that all the free encoders I'm aware of use the 180° phase inversion - hence, both left and right surround are in opposite phase of the front channels, while the surround left should be -90° and surround right should be +90°.
Usually there are not big differences, but for example when a circular pan happens, the +180° approach fails to reproduce it correctly.