Remove implementation of Externalizable from MatrixStack
Remove implementation of Externalizable from MatrixStack

Externalizable requires the class to implement a public no-args ctor,
and we do not want to do that, in order for a client to not invoke that,
which would leave the MatrixStack instance in an uninitialized state.
JOML-CI/JOML,JOML-CI/JOML,FortressBuilder/JOML,roquendm/JOML,roquendm/JOML,FortressBuilder/JOML,JOML-CI/JOML