[cairo] How to remove png's "magic pink"?

[Carlo]

Hi,
I'm wondering how I can remove the "magic pink" color (FF00FF) from a 
png. I've some png without alpha channel, they've a background color 
that I don't want to show.  I'd like to make a something such a color 
keying. There's a way to remove it? Perhaps using a mask and a certain 
cairo_operator? I didn't see an example of how to do this.

Thank you in advance
Carlo Ferraresi