Rendering web page to arbitrary port

[Wordman]

I'm interested in writing an application using an embedded browser that 
would allow me to capture an entire web page to a .bmp file. I want the 
_whole_ page as a single bitmap, even the bits that are scrolled off 
screen.

The whole process seems conditional on being able to get Mozilla to 
render into an arbitrary port. Can embedded Mozilla do this easily? If 
so, how is it done?