This is handled by calls to espeak_ng_Cancel/espeak_Cancel and
espeak_ng_Terminate/espeak_Terminate.

Removing these calls do not impact on the behaviour of the program
and simplify the wave.h API.

These calls are used to pre-emptively exit when writing audio
data. This is also handled within dispatch_audio. The difference
is that this callback could allow espeak to cancel quicker. In
practice it does not make much difference.

This call was used to update the written sample count used by
wave_get_remaining_time. As that function has been removed, this
code is no longer needed.

This logic was implemented to limit the number of audio events sent
to a device, but the logic is not too relevant in practice (these
calls return 0 most of the time), and the logic is broken (e.g. when
calling Synthesize with a very long block of text).

This function call does not affect the audio output in non-async
modes. Audio output in that case is handled within the SynthOnTimer
helper function.