Change `WorldToScreen` to be a static method.

`WorldToScreen` is declared `public bool` without `static`. The error message suggests that you are trying to call a non-static method without calling it for a specific object. You are calling `WorldToScreen` with `GameGui`. In C# naming convention, identifier starting with capital letter may refer to a class, a struct, an enum, a public property, or a public method. Without further information and based on the error message that you got, I assume `GameGui` is a class. Calling a non-static method using class name only would give the error that you encountered.
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