[plt-scheme] how does sgl flush graphics ?

I'm experimenting with sgl and having trouble figuring exactly how it
updates redraws - although this may be more of mred question...

1st of all it looks like double-buffering is "automatically" turned on.
true ?

what exactly is "with-gl-context" for ?  I assume it's so that the
underlying canvas widget knows that this is an open-gl call instead of
a wx-windows call.  true ?

are overlay frames accessible in sgl ?

Thanks

Brian