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!).

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

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.

Controller::WasKeyReleasedThisFrame() is now keyboard->key_released() so
there is usually no need for a controller. They will completely disappear
when joystick axis binding has been ported to InputDevice.

Most input and GUI files are now in a common ui/ directory. The platform/
directory is no longer, as everythyng was moved to application/ or ui/.

In particular, get rid of the enum magic; we can use map<string,int> if we
really need some named enums. Which we usually don’t.

The scene now owns its renderer, so there is no need to keep a list
of existing renderers and to call Renderer::Get().

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