Multi client support

[borine]

Permit more than one client to open each PCM. For playback clients, the streams from each client are mixed together before encoding. For capture clients each client receives a copy of the decoded stream.

Stream mixing is done in C code, there is no explicit SIMD code. However, compiling with -O3 does appear to allow gcc to optimize the mix loop very well on x86_64. I have not tested optimization with aarch64 (my old RPi machines are all 32-bit with no SIMD); but even without optimization I can run 3 playback clients simultaneously on a RPi zero W.

There was some interest in this feature three years ago, but there have been only a few enquiries more recently. I think this is pushing the scope boundary a little, as for most systems requiring multiple simultaneous audio applications there are also other requirements which mean that pipewire is probably a better fit. However I do have one genuine use case which is a "kiosk" system running only chromium which requires each chromium tab to independently open the PCM; and for this system BlueALSA with multi-client support is ideal.

[codecov]

Codecov Report

❌ Patch coverage is 63.78803% with 369 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.21%. Comparing base (29cfde6) to head (b41a35b).

Files with missing lines	Patch %	Lines
src/ba-pcm-client.c	53.46%	141 Missing ⚠️
src/ba-pcm-multi.c	71.83%	98 Missing ⚠️
src/ba-pcm-mix-buffer.c	59.57%	76 Missing ⚠️
src/main.c	0.00%	19 Missing ⚠️
src/asound/bluealsa-pcm.c	42.85%	16 Missing ⚠️
test/test-io.c	84.44%	8 Missing and 6 partials ⚠️
src/ba-transport-pcm.c	81.25%	3 Missing ⚠️
src/bluealsa-dbus.c	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #762      +/-   ##
==========================================
- Coverage   69.47%   69.21%   -0.26%     
==========================================
  Files         118      121       +3     
  Lines       17303    18291     +988     
  Branches     2739     2910     +171     
==========================================
+ Hits        12021    12660     +639     
- Misses       5162     5505     +343     
- Partials      120      126       +6     

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