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()

Automatic stream routing should only be used when the ma_device object
is initialized with a device ID of NULL. When it's non-NULL (i.e. an
explicit device) it should not perform automatic stream routing. It
looks like the WASAPI backend is not respecting this requirements and
is enabling automatic stream routing in all cases.

Public issue #127

The rationale for this is to test that the MA_NO_DECODING option is
compiling clean.