Allow application developers to modify cursors

With this change, cursors can be set on the map target again.
Add MultiPoint to modify example
Check geometry type instead of `instanceof`
Fix MultiPoint modify
Merge pull request #1689 from tonio/fix_modify

Check for geometries type instead of `instanceof`
Merge pull request #1684 from ahocevar/ie-cursor

Changing pointer cursor in examples is broken
Dispatch 'render' event in renderer.canvas.VectorLayer

The context property of the event only contains the vector rendering.
Add ol.layer.Heatmap class and example