This commit adds multi-track functionality without exposing it to the
user.
All I really did was rename AudioEngine to Track and allow more than one
Track in TrackManager. A lot of the changes are just changing widgets to
take a TrackManager and use current_track().
The TrackManager creates Tracks and gives them a read-only reference to
the global time value. When the TrackManager wants to fill a sample in
the buffer (in fill_buffer()), it calls fill_sample() on each Track.
The delay code is slightly different - a Track will fill its
m_delay_buffer with the sample it just created rather than the most
recent sample in the buffer (which used to be the same thing).
TrackManager manages the current octave.
Other than those few things, this is a pretty basic separation of
concerns.
This commit adds basic support for importing, viewing and playing WAV
samples at different pitches.
Naming issues:
- We are using the Sample struct from Music.h, but also the Sample
struct from LibAudio (Audio::Sample). This is a little confusing.
set_recorded_sample() finds the peak sample and then divides all the
samples by that peak to get a guaranteed min/max of -1/1. This is nice
because our other waves are also bound between these values and we can
just do the same stuff. This is why we're using Audio::Sample, because
it uses floats, whereas Music.h's Sample uses i16s. It's a little
annoying that we have to use a mixture of floats and doubles though.
For playback at lower frequencies, we're calculating in-between samples,
rather than just playing samples multiple times. Basically, you get the
current sample and add the difference between the current sample and the
next sample multiplied by the distance from the current sample. This is
like drawing the hypotenuse of a right-angled triangle.
