[rendering] fix/align ISM conversion between internal and external renderer

See tests/renderer/constants.py.

ISM rendering has differences between internal and external rendering in:

• crossfades
• rounding of ISM positions (decoder uses float + 0.5f to int16_t, see #215 )
• stereo downmix for discrete ISMs uses tangent panning; the external renderer uses EFAP
• ISM to BINAURAL and BINAURAL_ROOM rendering are also possibly affected by the above inconsistencies