Closes#3.
The code which defines what the buttons do and downloads songs and so
forth are now completely separated from the player, which has the
following API:
- `XMPlayer.initAudio` -> starts up audio context; it's available as
`XMPlayer.audioctx`
- `XMPlayer.loadXM(ArrayBuffer)` -> returns `true` if loaded, otherwise
barfs randomly
- `XMPlayer.playXM()` -> starts playing
- `XMPlayer.pauseXM()` -> obvious
- `XMPlayer.stopXM()` -> obvious; call this before loading a new one
Loading trackview.js is now optional; without it, the player won't do
any visualizations. Or, you can override the following to get callbacks:
- `XMView.pushEvent(e)` -> push an audio event onto the queue. Called
once per tick (about 50Hz, controlled by song). `e` contains fields:
- `t` - audio timestamp
- `vu` - Float32Array of RMS power (volume) for each channel
- `scopes` - [Float32Array] of oscilloscope data, one array per
channel; `XMView.scope_width` contains # of samples to produce here
- `songpos` - position in the song (# patterns played)
- `pat` - pattern number currently playing
- `row` - row within pattern
- `XMView.pause()` - pause visualization
- `XMView.stop()` - stop/reset visualization
