require openseadragon if it doesn't exist
require openseadragon if it doesn't exist

require openseadragon if it doesn't exist

Fixes the `[openseadragon-canvas-overlay] requires OpenSeadragon` error when doing this:

```
import OpenSeadragon from 'openseadragon';
import 'svg-overlay';
```

Borrowed style from https://github.com/usnistgov/OpenSeadragonFiltering/blob/master/openseadragon-filtering.js

setting OpenSeadragon object to `$`

Note, I requested a similar change in svg-overlay:
https://github.com/openseadragon/svg-overlay/commit/e7bfc6725b15ff5cfa7788f01a877964e9480baf#diff-d723f0aa2fac0a96816f277e6c2b17e2
altert/OpenSeadragonCanvasOverlay,altert/OpenSeadragonCanvasOverlay