* masking performance: avoid the repeated calls to `closest` when recursing through the DOM
- needsMask===true means that an ancestor has tested positively for masking, and so this node and all descendents should be masked
- needsMask===false means that no ancestors have tested positively for masking, we should check each node encountered
- needsMask===undefined means that we don't know whether ancestors are masked or not (e.g. after a mutation) and should look up the tree
* Add tests including an explicit characterData mutation tests
* Further performance improvement: avoid calls to `el.matches` when on a leaf node, e.g. a `<br/>`
---------
Authored-by: eoghanmurray <eoghan@getthere.ie>
Based on initial PR #1338 by Alexey Babik <alexey.babik@noibu.com>
