InputStreamAudio: suppress tmp variable
AudioDesc: include AudioFrame instead of forward declaration

AudioFrameDesc can be used in the public API.
AudioTransform: set options in a simpler way

Use swr_alloc_set_opts() instead of setting options one by one, with a
risk of runtime error if an option we want to set doesn't exist.
AudioRewrapper: refactoring

* Rename audioODesc to audioOutputDesc.
* Explain the magick numbers ;)