Rename “channels” to “tracks” when talking about the mixing channels, to
avoid confusion with channels in the context of mono/stereo/5.1. Also allow
conversions between various formats and frequencies.

To do: allow to query the current audio device’s preferred frequency when
the client application is able to send the best possible data.

This lets us get rid of m_autorelease and m_destroy and we’ll be able
to implement init_game/init_draw later, for stuff that must happen
on the render thread.

We now just look for SDL2/SDL.h (the regular location) and SDL.h (when
the user provides their own location for this header).

This was not a very smart class. We replace it with a bidirectional map.

This will allow us to generate sounds on the fly on several dedicated
channels. Until now we could only play fully loaded samples.