Bitmap outside a window

[Martin Bailey <martin pcalpha com>]

Hi,

Sorry for asking such a basic question; I'm simply unable to unable to 
find my answer in the documentation and this is my first attempt at gtkmm.

I'd like draw a Bitmap outside a Window. That is, I would like to draw 
an irregular black shape (from a bitmap matrix) directly in the display, 
over any other programs.
I don't want to draw inside a Window because I don't want my application 
to receive any events; just draw over other applications, or in a way 
hide some parts of them with a binary bitmap.

So as I understand, I must create a Bitmap in the same way that I create 
a Window, and then create a GC to draw inside my bitmap.
Is that right?
What C++ code do I need to get these elements created properly?

Best regards,
Martin