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.

Executables that use libbullet are now 1 MiB smaller (stripped!).

Devices were not properly reset at each frame, which caused
button_pressed() and other functions to fail.

Now we can call functions that need to access GPU resources explicitly
on the draw thread.