code cleanup: move runtime var zfac out of RegionView3D. rename initgrabz() -> ED_view3d_calc_zfac() and have it return the zfac to use.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55132 954f8c5b-7b00-dc11-b283-0030488c597c