Fixes:

* Overlay invalidation did not happen when changing brush, .either
through UI or through shortcuts
* Add initiliazation of curves before threaded overlay texture
evaluation or we may get memory leaks due to race conditions.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55997 954f8c5b-7b00-dc11-b283-0030488c597c