Fix: jittered brushes are not jittered, reported by kursad karatas.
Issue is sharing using global random generator which is shared with
particle system which resets the seed due to some scene/option
combination. Since it may be desirable to get predictable results with
particles, made sure brushes allocate their own random number generator
on startup and use that for jittering.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55418 954f8c5b-7b00-dc11-b283-0030488c597c