Fix #36905: backport of fix in soc-2013-paint. This commit ensures that during color sampling the cursor of the current brush is disabled. This avoids sampling of the brush cursor for really small brushes. Only caveat is that if operator exec is called then cursor might be visible in new sample location. This is not so common though.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60682 954f8c5b-7b00-dc11-b283-0030488c597c