a-Inline Spectrogram used a ringbuffer to send mixed down audio data
from the DSP thread to the inline display thread. The problem is that
several inline display threads can coexist (one for the channel strip in
the editor, one for the channel strip in the mixer, and soon one for an
inline display in the generic plugin UI). A ringbuffer is single-writer
single-reader so each display only got part of the data, and all were
competing for it.
Replace it with a circular table, where the DSP sets a write pointer,
and every (inline display) user keeps its own read pointer and checks it
is not so far in the past as to be overtaken by the DSP write pointer.
