This is the first step to getting rid of lol::array in favour of std::vector.

Does not work very well because of CORS restrictions in browsers.

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.

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