crash after adding a canvas to map (no error thrown)

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**mapbox-gl-js version**: 0.36.0
**mapbox-gl-draw version**: 0.17.0

### Steps to Trigger Behavior

Add a canvas source like:
```
map.addSource('heatmapsource', {
type: 'canvas',
canvas: 'heatmap-canvas',
animate: true,
coordinates: [
[west, north],
[east, north],
[east, south],
[west, south]
]
});
```
It does not matter if you add it before or after initialising Mapbox Draw

### Expected Behavior
It should work.

### Actual Behavior
It doesn't. All features disappear, when clicking a draw control, the cursor does not even become a crosshair, total f up. No error thrown :-(

Reproduce the failure with mapbox-gl-js 0.36.0, mapbox-gl-draw 0.17.0, the canvas source example, and both initialization orders. Compare the map with and without the canvas source, then verify that Draw controls remain usable, features stay visible, and no silent crash occurs.

Written by the indexing model from the issue text.