drawing a pixmap with transparent pixels

[Rob Chansky <robc titan com>]

Hi all. I'm trying to draw a pixmap onto another pixmap and have the 
first pixmap have transparent pixels so that the background of the 
second pixmap shows through.

I can see that the general rule is to draw onto my pixmap, and then draw 
onto a bitmap, and use that bitmap as a mask to display the pixmap. My 
problem is, I'm already CPU-bound and don't want to draw twice.. that 
would remove the margin I would gain from keeping cached pixmaps of the 
generated animation.

I can see that gdk_pixmap_create_from_data() loads both a pixmap and 
bitmap from a data file, and that's almost what I want. I can see that 
some pixbuf routines do what I want, too, but my problem is I'm drawing 
onto a pixmap using gdk_draw_arc() and ilk and I can't draw those onto a 
pixbuf... and copying my pixmap into a pixbuf (across the server 
barrier) and back won't get me my optimization either...

Is there a way to convert my pixmap into a bitmap and keep it on the 
server side?  Or, failing that, can I use gdk_draw_arc() etc onto a pixbuf?

Rob