Draft: [Non-BE][rend-non-BE][split-non-BE] Resolve "Head tracking API causes extra 15ms of delay"

Related to #500 (closed)

Proposal to solve the additional head tracker delay, change the API to go always through the JBM path to allow for 5ms rendering. As side effect this will also remove some duplication of functionality which already created some issues in keeping the JBM path and the non-JBM path in sync.

Not yet completely BE, probably won't be BE in the end, because for the external orientation at least I do not see a way to keep BE.

The code also contains a few temporary fixes to make BE tests to the baseline possible

ToDos:

• check if external orientation BE is possible
• try to fix remaining BE issues that are not related to the external orientation
• mem optimization
• MC ParamUpmix (!767 (merged))

A review by @pihlajakuja would be very much appreciated.

Also contains fixes for #551 (closed) (Non-BE), #676 (closed) (BE), #591 (closed) (Non-BE), #678 (closed) (BE), and #679 (closed) (Non-BE)