fix for bug where edge slide could set mouse start/end points to the same value - so you couldnt move the edge loop at all,
now check that the screenspace projection of the start/end points have some length and fallback to fixed offset if none do.

also rename confusingly named vars.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56164 954f8c5b-7b00-dc11-b283-0030488c597c