Old bevel 'Clamp overlap' code was very naive: just limit amount
to half edge length. This uses more accurate (but not perfect)
calculations for the max amount before (many) geometry collisions
happen. This is not a backward compatible change - meshes that
have modifiers with 'Clamp overlap' will likely have larger allowed
bevel widths now. But that can be fixed by turning off clamp overlap
and setting the amount to the desired value.
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