When the desired widths (offsets) of beveled edges cannot be
satisfied, often because we want them to meet on an intermediate
non-beveled edge, we need to compromise on the widths somehow.
This code changes the compromise to minimize the sum of squares
of errors in the offsets. It also adds a global width consistency
pass: starting from a vertex that needed width adjustment, it
uses a breadth-first search to try to propagate the adjustments
and keep the bevel widths from having to taper along the edges.
Also fixed a case where a reflex angle would cause bad results.
Also fixed the way the 'percentage' width method was calculated.
da91575206