Add a gain control API.

A new op_set_gain_offset() allows the application to provide its own
 offset to the current decoder gain setting, as well as specify what
 offsets should be applied.
The header gain alone is still the default, but the application may
 also request that the track gain be applied, or that neither be
 applied.

In addition, an op_get_track_gain() function can parse the track
 gain out of a set of comment tags.
This is mainly provided as a convenience for applications that need
 this information, so they don't have to write their own parser.