Hi,
Here is a patch (that apply against the official CVS repo) with a minor correction and a test case (implemented in the clim-fig example) of XRender rectangle opacity.
I'd like to make other routines (medium-draw-polygon*, medium-draw-ellipse*,...) to use the XRender extension. So far, I've tested cl-geometry (https://github.com/Ramarren/cl-geometry) that can decompose a polygon as triangles that can then be feed to xlib:render-triangles-1. Is there any advice (other than "use the gtkairo backend")?