* Sounds can now have another node be used as input rather than a
    data source.

  * Sounds can now have a configurable input and output channel count.
    For sounds that are backed by a data source, the input channel
    count will always be set to the data source's native channel count.

  * Sounds can now be initially attached to any node and input bus
    rather than only a sound group.

With this change, sounds can now be used as groups. In future commits,
it's likely that ma_sound_group will be unified with ma_sound. Whether
or not the `ma_sound_group_*()` APIs will continue to exists is for now
undecided.

This change allows more flexibility for doing custom effects before the
spatialization stage in the DSP pipeline. The problem with the existing
design is that there's no way to apply a custom effect before
spatialization which becomes a problem because spatialization will
often increase the channel count which results in excessive effect
processing due to the increased channel count. Now it should be
possible to set up a graph such that an effect can be plugged in
between the data source and the spatializer.

A new function called `ma_sound_init_ex()` has been added which is what
needs to be used to initialize a sound without a data source. This API
uses the config/init pattern. The config is called `ma_sound_config`.