Blender Git Loki

Sculpt: Clay Strips Tip Roundness property

This patch allow to change the brush tip shape between a square and a
circle using a brush property.

After this change we are no longer testing the distance against a cube
(the Z axis is not used). I did not test this in depth, but if it does
not produce any artifacts I think we can keep it this way instead of
adding more complexity to the code.

In this new distance test the brush falloff is only applied on the
rounded parts of the square to avoid sharp artifacts in the
diagonals. Because of this, the round version is much softer than
the square one. The planned hardness property will fix this, but
this can also be avoided by setting the fallof to a custom curve.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D6165