This is in preparation for some work coming to the upcoming resource
manager.

This was not setting the NON_BLOCKING flag correctly when NO_THREADING
is set.

This is useful for platforms that do not have support for threading or
programs that do not want to use an extra thread for resource
management and would rather process jobs manually.

When configuring the resource manager to not use threading, the
MA_RESOURCE_MANAGER_FLAG_NO_THREADING flag must be set in the config.
This implicitly enables the MA_RESOURCE_MANAGER_FLAG_NON_BLOCKING flag
because it requires programs to manually call
ma_resource_manager_process_next_job(), and since it's assumed that
won't ever be called from another thread, you'd never want that to be
blocking.

This sets up a framework for getting the resource manager working with
Emscripten.

This needs a rethink. My test is no longer looping. Needs further
investigation.

This is an optional callback and is used by miniaudio as a hint to help
it determine how many input frames to read at a time. Without this,
miniaudio needs to guess how many frames to read, and in certain
situations may end up overestimating. This callback is only useful for
nodes that process input and output frames at different rates, i.e.
nodes that perform resampling.

This saves a mandatory call to ma_audio_buffer_ref_set_data(). With
this change, an ma_audio_buffer_ref_init() call is all that is required
to initialize a usable data source.

This is a data source whose backing data is an application-controlled
pointer. No data is copied. It's a way of efficiently wrapping a raw
buffer and using it as a data source.

Change of mind on how this should work.

This is useful for retrieving information about some aspect of the
node. A good example is human readable names associated with the node
and it's input and output buses. This is useful for user interfaces
where a brief description of the node such as "Low Pass Filter" can be
drawn on the screen. It's also useful for buses to be named, such as
the source/carrier and excite/modulator on a vocoder effect which would
also need to be visible on a UI.

The following flags can now be associated with nodes via the vtable:

  * MA_NODE_FLAG_PASSTHROUGH
  * MA_NODE_FLAG_CONTINUOUS_PROCESSING
  * MA_NODE_FLAG_ALLOW_NULL_INPUT
  * MA_NODE_FLAG_DIFFERENT_PROCESSING_RATES

See commit changes for a description of these flags.

  * The simplified callback has been removed.
  * The `globalTime` parameter has been removed from the callback.
  * The order of input and output frames and counts has been swapped to
    be consistent with ma_data_converter_process_pcm_frames(), etc.

Public issue https://github.com/mackron/miniaudio/issues/260

This enables the ability to explicitly enable only the backends a
program is interested in. Essentially it's the reverse of the pre-
existing method whereby instead of disabling backends, all backends are
disabled by default, and then specific backends are enabled. Example:

    #define MA_ENABLE_ONLY_SPECIFIC_BACKENDS
    #define MA_ENABLE_WASAPI /* Only care about WASAPI on Windows. */
    #define MA_ENABLE_ALSA   /* Only care about ALSA on Linux. */

Note that even if MA_ENABLE_* is used, the backend will still only be
enabled if the compilation environment and target platform actually
supports it. You can therefore use the MA_ENABLE_* options without
needing to worry about platform detection.

Public issue https://github.com/mackron/miniaudio/issues/260

Variables are marked with this annotation to make it clear that access
to the variable should be done through atomics.

I've also review the use of volatile in this commit.

Public issue https://github.com/mackron/miniaudio/issues/259

This was setting the previous pointer of newly attached nodes to NULL
instead of a pointer to the dummy head node. This then results in the
dummy head node never being updated when the node is detached which in
turn results in an uninitialized node being dereferenced.

This would try reading from an uninitialized buffer thereby resulting
in a bad audio glitch. This does two things to fix the problem:

  1) When there are no input nodes attached to an input bus, nothing is
     read and 0 will be returned for the frames read variable.

  2) The buffer is silenced by default.

This fixes a bug with ma_engine where it would glitch in the moment
just after the engine is initialized and before a sound or group is
attached.