[dsp] add spatial instrument support

[anokta]

can support multichannel sample playback
can implement internal panning and spatialization

[anokta]

current state:
fully switched internal processing for stereo
zero-filling rest of the channels if output has more channel (or downmix to mono if single channel)
future:
can support surround etc properly at some point in the future (and potentially increase the number of internal processing channels for other panning/spatialization features)