transparent glass layer over windows

[Martin Portman <mrp cre canon co uk>]

hi (from a relative newbie),

i want to put a transparent 'glass' frame over a window, to intercept
mouse events, possbily draw on the glass, and pass mouse events on to
what's below the glass.

think of a gesture recogniser, receiving mouse events/signals, drawing
the stroke, but sending key events down, and removing the stroke.

in java i'd use the glassPane part of root panes to do this.

how do i do this using gtk?   

thanks,
martin.