PVector.limit() does not behave as expected with 2-D Vectors

Reported on ZenDesk.

"If .add is used with only two parameters (i.e., a 2-D vector) and then .limit is used, the .limit function has no effect. If the .add is used with 0 for the third parameter, everything is fine."

Locate the PVector.limit() and two-argument PVector.add() entry points, then reproduce the reported difference between a 2-D add and an add with a zero third parameter. Trace why limit has no effect in the 2-D case and verify that both forms produce consistent limiting behavior.

Written by the indexing model from the issue text.