Sculpt: Split original normal into original normal and plane

This allows to create different effects with some brushes that use the sculpt plane.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D5818

source/blender/makesdna/DNA_brush_types.h

@@ -406,7 +406,7 @@ typedef enum eBrushFlags {
   BRUSH_SIZE_PRESSURE = (1 << 3),
   BRUSH_JITTER_PRESSURE = (1 << 4),
   BRUSH_SPACING_PRESSURE = (1 << 5),
-  BRUSH_FLAG_UNUSED_6 = (1 << 6), /* cleared */
+  BRUSH_ORIGINAL_PLANE = (1 << 6),
   BRUSH_GRAB_ACTIVE_VERTEX = (1 << 7),
   BRUSH_ANCHORED = (1 << 8),
   BRUSH_DIR_IN = (1 << 9),