Adjust leaflet-image to work with leaflet 0.8-dev

`layer._adjustTilePoint()` is removed in leaflet master version. `map._pathRoot` is moved to a pane.

Note that `L_PREFER_CANVAS` is also removed, so we need to add the path-layers with a canvas renderer:

```JavaScript
var canvas = L.canvas({pane: 'overlayPane'});
L.polyline([[1, 2], [4, 5]], {renderer: canvas}).addTo(map);
```