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Fix T85079: Thumb brush unstable when producing larger displacements

The thumb brush was updating the area normal per brush sample, which was
making unstable sampled normals for setting the displacement direction
when the vertices are moved too much from their original positions.
Now it always uses the original normal except when using anchored
strokes. In those cases, the normal always needs to be updated.

Reviewed By: JacquesLucke

Maniphest Tasks: T85079

Differential Revision: https://developer.blender.org/D10214