Public issue https://github.com/mackron/miniaudio/issues/265

Public issue https://github.com/mackron/miniaudio/issues/258

These are in preparation for some spatialization functionality.

This does not yet support the following:

  * Cone based attenuation
  * Doppler effect
  * Multiple listeners

This had the naming of source and excite the wrong way around.

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.