min/max constraints cause out of bounds freezing

[isusanin]

Steps to reproduce:

1. create a circle at (0, 0)
2. create a variable called slider and initialize to 0.0
3. set X of circle to min(5.0, max(0.0, slider))
4. Set control of circle to X/slider

Now drag the circle quickly. You'll notice that you can easily get the slider value below 0.0 or above 5.0, violating the constraint. When this happens, the circle also gets stuck and you can't drag it anymore.

[electronicwhisper]

I'm going to mark this a duplicate of issue #12, constraining variables within bounds.