[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
Edited by Archit Tamarapu