Don't allow nudging delta to grow past min/max clipping points

[peterflynn]

1. Start with a value like #abc
2. Nudge up (Shift+Alt+Up) until you hit #fff
3. Nudge up five more times
4. Nudge down

Result: nothing changes. You have to nudge down through five no-ops before the value starts changing again.

This is because the nudge code keeps inc/decrementing the scrubState delta past the clipping point, since it has no knowledge of what clipping is going on. This behavior makes sense for mouse dragging, but not for keyboard nudging.