add clip_segment_v3_plane_n() to clip a line segment to planes (as used for view clipping).
use in ED_view3d_win_to_segment_clip() and fix error, was clipping by only 4 planes rather then 6.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56219 954f8c5b-7b00-dc11-b283-0030488c597c