Call loadFeatures from ol.source.ImageVector

This PR fixes a bug [reported](https://groups.google.com/d/msg/ol3-dev/sScaKKx2744/8qSFCC2VZY8J) on the mailing list that makes it impossible to use an `ol.source.TileVector` in an `ol.source.ImageVector`.

The fix involves calling `loadFeatures` from `ol.source.ImageVector#.canvasFunctionInternal_`, as done by the canvas vector layer renderer.

Please review.