balls demo may raise constraint_error

[rveenker]

Hello,
I found that the balls demo will at some point raise constraint_error. Especially when there are a lot of balls :-)
The problem arises when the center coordinate of a ball gets a negative component from Handle_Collision. When the center coordinate is converted to a point which has values X and Y of type Natural, the constraint_error occurs. This happens when a call is made to Fill_Circle.

[lambourg]

I knew that my collision algorithm is not right ;) Honestly, this balls demo is not the one I run the most, but I'll try to make some effort improving it. I'll have a look.