[plt-scheme] MysterX and ImageMagick COM object

> This avoids attempting a direct call through the COM function 
> pointer, when a vararg function is being invoked.

I'm not sure that's necessary.  As the IDL shows, there's a 
fixed number of arguments to push on the stack -- a pointer to a 
SAFEARRAY.

> The change here calls 
> buildMethodArgumentsUsingDefaults if a vararg function is 
> being invoked.

This approach smears the arguments into the vector of arguments 
used by IDispatch::Invoke().  Does MS say this is what to do 
SAFEARRAYs in general?  

-- Paul