Unlikely fix for https://github.com/dr-soft/miniaudio/issues/134

The ma_waveform API is a more general API supporting different waveform
including sine, square, triangle and sawtooth.

This has been renamed to ma_calculate_frame_count_after_resampling()
for consistency with the new ma_resampler API.

This previously returned the frame count based on the internal sample
rate of the underlying stream, whereas it should have instead returned
a frame count based on the output sample rate. This commit fixes this.

This commit commit also changes the implementation of
ma_calculate_frame_count_after_src() to use the ma_resampler API which
should make it a bit more robust and easier to maintain.

Public issue https://github.com/dr-soft/miniaudio/issues/126

This change makes a copy of the master volume factor before applying it
so that if another thread changes the volume it won't apply the change
part way through the processing of a period.

With this change the stop callback must be set via the device config.

This example is no longer useful as everything is explained in the
documentation of ma_context_init() and ma_device_init().