[plt-scheme] mred object -> wx C++ object

Hi all,

If I pass a MrEd object (say a bitmap-dc%), to a C/C++ extension, how  
can I get the pointer to the wx C++ object from the Scheme_Object*?   
I ask because I wrote some image processing code, and I want to see  
what kind of speed improvement I get by pushing it into C/C++.  I'm  
digging around in this MrEd/wx code and getting lost.

thanks,
Rob