Compatibility and robustness:

* Make masking depend on texture mode only if there's an actual texture
present
* New stroke system uses diameter to calculate stroke spacing, while old
texture painting brushes and system used radius. So divide spacing of
brushes in old files to half. Since I expect trunk users to have already
adapted this, I do this for an old subversion of blender.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55911 954f8c5b-7b00-dc11-b283-0030488c597c