Need a consistent scheme for enforcing invariants such as range checking

When we move to lazy evaluation for all scene graph operations (RT-5483), we will have to rethink the way in which we enforce scene graph variants. The FX language makes enforcing invariants impossible in the general case anyway. One possibility is to let the user-settable variables be whatever the user sets them to be, and then dealing with them appropriately at usage time, either by clamping, or ignoring illegal values.