dep: Add cubeb

dep/cubeb/TODO

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+TODO:
+- directsound: incomplete and somewhat broken
+- osx: understand why AudioQueueGetCurrentTime can return negative mSampleTime
+- test (and fix) sub-prefill size data playback
+- report stream delay instead of position; leave position calculation to user
+- capture support
+- capture and output enumeration and configuration
+  - also expose default hardware config to allow decisions on speaker layout
+- prefill occurs at different times in each backend:
+  - pulse prefills async off worker thread after init
+  - coreaudio prefills during init
+  - alsa prefills async after start
+- expose configured prefill size; may differ from requested latency
+  - solved by exposing stream delay
+- xruns may occur in user callback but also in audio hardware
+  may need to expose details of hardware xruns to user api
+- document thread safety
+- document which calls may block, and when effects take effect
+- document what's permissible inside callbacks
+- implement basic channel mapping for surround
+  - vorbis has documented mapping based on channel count (if mapping type ==
+    0) -- http://xiph.org/vorbis/doc/Vorbis_I_spec.html#x1-800004.3.9
+    1 -> M
+    2 -> L, R
+    3 -> L, C, R
+    4 -> L, R, RL, RR
+    5 -> L, C, R, RL, RR
+    6 -> L, C, R, RL, RR, LFE
+    7 -> L, C, R, SL, SR, RC, LFE
+    8 -> L, C, R, SL, SR, RL, RR, LFE
+   >8 -> application defined
+  - wave files with channel count only
+    3 -> L, R, C
+    4 -> L, R, RL, RR
+    5 -> L, R, C, RL, RR
+    6 -> L, R, C, LFE, RL, RR
+    7 -> L, R, C, LFE, RC, SL, SR
+    8 -> L, R, C, LFE, RL, RR, SL, SR
+  - wave files with WAVE_FORMAT_EXTENSIBLE have explicitly mappings, can
+    extract these
+- implement configurable channel mapping