Go back to a regular function pointer for the assertion callback.
Go back to a regular function pointer for the assertion callback.

I had just switched to using std::function, but function values cannot
be compared, making testing more complicated.  (I didn't notice at
first because VS2012 apparently does implement operator== for
std::function, but it's not in the standard, is not supported by other
compilers, and we shouldn't use it.)

simquest/opensurgsim,simquest/opensurgsim,simquest/opensurgsim,simquest/opensurgsim