audio: Implement cubeb audio out backend. (#1895)

* audio: Implement cubeb audio out backend.

* cubeb_audio: Add some additional safety checks.

* cubeb_audio: Add debug logging callback.

* audioout: Refactor backend ports into class.

* pthread: Bump minimum stack size to fix cubeb crash.

* cubeb_audio: Replace output yield loop with condvar.

* common: Rename ring_buffer_base to RingBuffer.

externals/CMakeLists.txt

@@ -228,6 +228,16 @@ if (NOT TARGET stb::headers)
     add_library(stb::headers ALIAS stb)
 endif()
 
+# cubeb
+if (NOT TARGET cubeb::cubeb)
+    option(BUILD_TESTS "" OFF)
+    option(BUILD_TOOLS "" OFF)
+    option(BUNDLE_SPEEX "" ON)
+    option(USE_SANITIZERS "" OFF)
+    add_subdirectory(cubeb)
+    add_library(cubeb::cubeb ALIAS cubeb)
+endif()
+
 # Apple-only dependencies
 if (APPLE)
     # date