This commit removes the custom data loop and instead implements the
onDeviceRead and onDeviceWrite backend callbacks. This simplifies the
implementation and fixes a bug with duplex mode where the capture and
playback devices have different native sample rates.

This commit also fixes a bug with fixed sized callbacks with duplex
mode.

Public issue https://github.com/mackron/miniaudio/issues/397

Public issue https://github.com/mackron/miniaudio/issues/400

Public issue https://github.com/mackron/miniaudio/issues/400

Public issue https://github.com/mackron/miniaudio/issues/399

Public issue https://github.com/mackron/miniaudio/issues/318

Public issue https://github.com/mackron/miniaudio/issues/318

Add support for GDK (<https://github.com/microsoft/GDK>)

Public issue https://github.com/mackron/miniaudio/issues/396

This is WIP. There's still some atomicity errors in here.

Public issue https://github.com/mackron/miniaudio/issues/318

A device job thread is used for processing jobs from a separate thread
for cases where doing a certain operation needs to be done from outside
a callback.

This is in preparation for generalizing the job system.