Added conversion method to Color class to avoid Debugger popup

Hi Dan,

After building a Squeak vm with the Gezira bindings, I attempted to run some of the examples and got a debugger popup.  Gezira was trying to send the "asGeziraFill" message to Color objects.  This message was implemented for Forms, but not colors. I implemented it as an extension method, and simply had the color create a "uniform" GeziraFill, using itself as the argument. After this, the examples ran fine. Please let me know if you require additional changes.

Thanks,
Jeff G.