[Bf-committers] Sculpt stroke questions

[Antony Riakiotakis]

I just committed the mentioned fixes. In my tests, the brushes worked much
better in the problematic use cases.

* Sculpt plane takes front facing into account only
* There is a category of front face only brushes, which right now only
includes the clay strips brush but may also include others (or remove it
altogether if real use cases for back-face sculpting exist) later,
depending on user feedback.