allow gmult to be overriden again

You removed the function `transformedGmult` in the following commit https://github.com/schteppe/p2.js/commit/c7d668d87517d87a38e8bfda586cf1c37b70bba2

But you use this function it in the motor of a prismatic PrismaticConstraint. As you changed it to a closure it's not accessible in the overwritten version. I made this function available again by declaring it as a prototype member instead of a private closure. Are you okay with this ?