Tap to add polygon vertex on tablet devices

[ahb108]

It would be good to enable tapping to add vertices for the polygon in OpenLayers. For an example that does work on a tablet, at least for feature editing, see:

http://ol3js.org/en/master/examples/modify-features.html

[ahb108]

This seems to be a problem with the way things are currently set up in template.html or possible due to its interaction with PyBossa. What is the distinction between these two calls to ol js source in template.html?
`<script src="http://ol3js.org/en/master/build/ol.js" type="text/javascript"></script>

<script src="/static/js/vendor/ol3photomask.js" type="text/javascript"></script>`

More generally, touch-based digitising works well on simpler implementations of OpenLayers3, such as the one I implemented here:
http://www.ucl.ac.uk/~tcrnahb/tmp/palsmall.html