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.

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

All these changes will allow us to more easily add Lol Engine as a
submodule of another Git project.

tested wherever relevant.

sub-millisecond but can be improved if we get rid of SDL timers.