fix bug in clipping

-This line, and those below, will be ignored--

M    Common/LuaBindings.cpp
M    Geo/MVertex.cpp
M    Geo/GVertex.h
M    Geo/MVertex.h
M    Solver/dgAlgorithm.cpp
M    Solver/TESTCASES/ForwardFacingStep.lua
M    Solver/dgSlopeLimiter.cpp
M    Solver/dgSystemOfEquations.cpp
M    Solver/dgConservationLawPerfectGas.cpp