draw new data after a context has been stopped and then started

This fixes issue #74.

I'm not quite sure why `cubism_metricOverlap` is subtracted, so I left it there.

I tested it with just 

```javascript
i0 = width -dx
```

and it seemed to work ok, except I once saw a single unfilled vertical pixel line, so maybe it's best to at least have 

```javascript
i0 = width - dx - 1
```