Blender Git Loki

Fix T49848: bevel of spiral gets bigger and bigger widths.

This is the same issue as was fixed with T39486: the adjustment pass
that tries to equalize different widths at either end of an edge
sometimes causes the widths to get bigger and bigger.
The previous fix was to let "clamp_overlap" do double duty as a way
to limit this behavior. But clearly this is undiscoverable, as the
current bug report shows. So I put in an "auto-limiting" mode that
detects when adjustments are going crazy and then acts as if
clamp_overlap were set.
The reason we can't always act as if clamp_overlap is set is that
certain models (e.g., Bent_test in regression tests) look bad if
that is enabled.