Formatting a string on a variadic parameter function without using GC

[David G. Maziero via Digitalmars-d-learn]

Yes, I'm aware of the null-termination thing. I might have pasted 
code that I already changed.

But I already messed with sformat, and it seems that it does use 
the GC. I've put @nogc in RenderText, and the compiler says 
sformat uses GC, so I don't know.

But the thing is, I don't want to build the string before calling 
RenderText, I want RenderText to do it.