Blender Git Loki

Fix brush tip delta orientation with anchored strokes

When using anchored stroke, the stroke operator was modifying the
coordinates on the "mouse" rna property by setting them to the original
position. Because of this, all the sculpt delta calculation was failing
and the delta for these brushes was set randomly (with a 0 vector) at
the beginning of the stroke.
There is now an extra property that uses the unmodified coordinates of
the mouse to calculate the delta. Now delta orientation works as expected
in all brushes and features that require brush tip orientation.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9183