This change reduces memory usage by removing half the number of delay
registers.

This commit also sets up some infrastructure for future work on a fixed
point implementation. Both the floating and fixed point implementations
will run on a 32-bit pipeline for now.

  * ma_convert_pcm_frames()
  * ma_sine_wave_read_pcm_frames()

This changes the the lpfCutoffFrequency config variable from a uint32
to a double. This is required because we simplify the sample rate
fraction which cant result in cases where the cutoff frequency is too
aggressively rounded down due to dividing tiny integers.

This is equivalent to passing in zeroes.

Public issue https://github.com/dr-soft/miniaudio/issues/128

This commit also renames ma_resampler_process() to
ma_resampler_process_pcm_frames().

  * ma_resampler_get_required_input_frame_count()
  * ma_resampler_get_expected_output_frame_count()

These have not yet been implemented for the Speex backend.

This commit also adds an APIs for setting changing the rate:

  * ma_resampler_set_rate()
  * ma_resampler_set_rate_ratio()