Fix PortAudio streaming and interruption.

[zeehio]

Rewrite the PortAudio engine so it works closely to the audio model in mimic.
It fixes the streaming capabilities* of mimic when using PortAudio.

Ported from MycroftAI/mimic1#137

* In case you didn't know, mimic is able to play as soon as samples start being generated. A custom function can be written (example here) to get a direct pointer to the sample data. The linked example plays the wave file, but it could simply memcpy it to a buffer if desired... That function is called right after each frame is synthesized (e.g. for clustergen voices it's called here)

[codecov-io]

Codecov Report

Merging #10 into development will decrease coverage by <.01%.
The diff coverage is 0%.

@@               Coverage Diff               @@
##           development      #10      +/-   ##
===============================================
- Coverage        17.82%   17.82%   -0.01%     
===============================================
  Files               80       80              
  Lines             8874     8876       +2     
===============================================
  Hits              1582     1582              
- Misses            7292     7294       +2

Impacted Files	Coverage Δ
src/audio/au_streaming.c	0% <0%> (ø)	⬆️

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