The sample format of a WAV file is not always being set which results
in get_data_format() returning ma_format_unknown.

This change makes use of the onInitFile, onInitFileW and onInitMemory
backend callbacks which enables decoding backends to have optimized
implementations for reading from a file or a block of memory without
having to go through an abstraction layer on the miniaudio side.

Public issue https://github.com/mackron/miniaudio/issues/696

Public issue https://github.com/mackron/miniaudio/issues/707

Public issue https://github.com/mackron/miniaudio/issues/700

When this is used, the data callback should at some point call
ma_engine_read_pcm_frames() in order to do some processing.

This is optional and is fired at the end of each call to
ma_engine_read_process_pcm_frames(). The callback will be passed the
processed audio data so they can do their own processing such as
outputting to a file or whatnot.

The callback is configured via the engine config.

With this change, dr_wav is now namespaced with "ma" which means dr_wav
can now be used alongside miniaudio.

In addition, some duplicate code has been removed, such as sized types,
result codes, allocation callbacks, etc. which reduces the size of the
file slightly.

This should address the following public issue:
  https://github.com/mackron/miniaudio/issues/673

miniaudio is updating it's amalgamation of dr_wav, etc. so that it's
all namespaced with "ma" which will make the amalgamated versions of
dr_libs entirely independent. There's no longer any need to mention
the decoding backends.

Documentation regarding stb_vorbis is removed so as to discourage
new users from using it. Support will not be removed until a
replacement Vorbis decoder can be amalgamated, but new users should
instead be guided to the libvorbis custom decoder in the extras folder.