WASAPI: Refactoring to simplify data reading and writing.

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