Fix #34818: Sculpting Mode only rotates around geometry origin (bring bug back)

Made it so average coordinate of previous stroke is used as
a viewport rotation center when Rotate Around Selection is
enabled in user preferences.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56168 954f8c5b-7b00-dc11-b283-0030488c597c