Public issue https://github.com/mackron/miniaudio/issues/942

This is in preparation for splitting miniaudio into a split .c/h pair,
away from a single header.

`MINIAUDIO_IMPLEMENTATION` is still supported, but will be removed in
version 0.12 and should be considered deprecated. It's recommended to
start the transition to the new .c file.

Revert "Fix for NDK issue 360. dlclose() skipped pre-API 28. This fixes weird crashes during uninit."

This reverts commit 14f698fcf289261946aefd0000dc5989062cf423.

The data source implementation of a ma_pcm_rb could possibly return a
frame count of 0 which would in turn result in
ma_data_source_read_pcm_frames() returning MA_AT_END which does not
make sense for a ring buffer since it has no notion of an end.

There is no notion of an "end" in a ring buffer. Also, this result is
returned when the operation completed successfully which makes a result
code other than MA_SUCCESS confusing.

With this commit, when targeting pthreads with the -pthread, the engine
will allow threading with it's internal resource manager.

Public issue https://github.com/mackron/miniaudio/issues/855

Support for this was added to Emscripten 3.1.70. Currently only
querying the quantum size is supported. I believe support for
configuring the quantum size is coming later in Web Audio 1.1.

Compilation with versions of Emscripten earlier than 3.1.70 is still
supported.