[rendering] fix/align multichannel rendering between internal and external renderer

See tests/renderer/constants.py

MC rendering has differences between internal and external rendering:

• MC-MC format conversion seems to have differences with high SNR, could be due to crossfading
• MC to STEREO downmix differs slightly
• MC to MONO downmix needs alignment
• Application of headrotation may not be consistent
• LFE is not correctly aligned to the binaural output signal, see #59 (closed) #110 (closed) #194 (closed)