Add support for custom backends.

This commit includes a few changes required for better supporting this:

  * Extra members have been added to ma_device_info which are going to
    eventually replace the min and max channels and sample rates. The
    new system is going to provide a list of supported data formats,
    groups by format/channels/rate and some flags. The only flag used
    at the moment is whether or not the format is usable in exclusive
    mode. The custom backend is the only backend currently using these
    new device info properties, and a backwards-compatibility layer has
    been implemented to fill out the old properties. Built-in backends
    will be migrated over to the new system in time.

  * A new set of backend callbacks have been implemented. Only the
    custom backend is using these at the moment. Built-in backends will
    be migrated over to these new backends soon.

  * A new public API called ma_device_get_state() has been added which
    returns the current state of the device (whether or not it's
    started or stopped). This is necessary for some custom backends.

  * A new public API called ma_device_handle_backend_data_callback()
    has been added. This is required for custom backends who use the
    callback paradigm for data delivery.

  * A new type of ring buffer has been created called ma_duplex_rb.
    This is used as an intermediary buffer for duplex devices running
    on backends that use the callback paradigm. It's used internally by
    ma_device_handle_backend_data_callback(). In the future it's
    planned to expand ma_duplex_rb to handle desyncs by dynamically
    resampling to get both sides back in sync. This is not implemented
    as of this commit.

Future work will involve converting existing built-in backends to be
consistent with the new ideas introduced with custom backend support.