Convert paint stroke code to write region instead of screen coordinates
on mouse event coordinates. Every paint mode operated on and converted
back to region coordinates, texture paint does so too so this will help
in unifying the systems.

ALERT! POSSIBLE BREAKING COMMIT:

I have tested all paint systems that use the stroke code and they look
like they work well but I would appreciate it if more eyes could test
this.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55032 954f8c5b-7b00-dc11-b283-0030488c597c