It notably simplifies things to mix HRTF sources into an accumulation buffer
together, which the Dry buffer's Ambisonic-to-HRTF decode is then added to,
before being mixed to the Real output.
It's somewhat ambiguous what they mean. Sometimes acting as a pointer, other
times having weird behavior. Pointer-to-function types are explicitly defined
as such, whereas uses of these tend to be as references (never null and not
changeable).
This puts the base coefficients and the phase deltas next to each other. This
improves caching, as the base and phase deltas are always used together while
the scales are only used for the non-fast versions.
This takes advantage of the fact than when increment <= 1 (when not down-
sampling), the scale factor is always 0. As a result, the scale and scale-phase
deltas never contribute to the filtered output. Removing those multiply+add
operations cuts half of the work done by the inner loop.
Sounds that do need to down-sample (when played with a high pitch, or is 48khz
on 44.1khz output, for example), still go through the normal bsinc process.
