Blender Git Loki

Fix Bevel percent mode, and clamping for it too (T79898).

The code for Bevel's percent (and absolute) modes were pretty bogus.
It assumed, like the rest of the modes, that the offset lines are
parallel to the beveled edge. Which is not true for these modes,
though it accidentally works sometimes if the legs are equilength.
Also the clamping code for those modes was completey wrong.
It is too hard to really fix the clamping code for absolute mode,
but it is a little better now. Percent mode clamping is fixed.