how to dynamically create a bitmap mask

["Stéphane Charette" <stephanecharette gmail com>]

Disclaimer:  I'm new to X, GTK+, and gtkmm...

I'm looking at the wheelbarrow.cc example
(http://www.gtkmm.org/docs/gtkmm-2.4/examples/window/wheelbarrow.cc)
to see how a mask can be created, and how it gets used in the call to
Gtk::Widget::shape_combine_mask().

What I would like, is to dynamically create a mask on-the-fly.  I'm
trying to modify the clock example
(http://www.gtkmm.org/docs/gtkmm-2.4/examples/book/drawingarea/clock/clock.cc)
to generate the right mask (a circle) when it starts up.

Problem is -- I cannot figure out how to dynamically create a
Gdk::Bitmap.  There are 2 Gdk::Bitmap::create() methods, but both
require a pre-existing XBM image.

Any help greatly appreciated.  Thanks!

Stéphane Charette