[Render] Howto avoid black rect outside of the drawable when compositing?

[Clemens Eisserer]

Hello,

I am trying to draw a transformed image using XRender, which works
quite fine, however the problem I experience is that pixels which are
outside of the src-picture are rendered black:
http://picasaweb.google.com/linuxhippy/Transformations/photo#5223368701623218514

The XRender-protocol-spec says:
> When fetching pixels from the source or mask pictures, Render provides three
> options for pixel values which fall outside the drawable (this includes
> pixels within a window geometry obscured by other windows).
> +	Transparent.  Missing values are replaced with transparent.
However I was not able to figure out howto set "it" to "transparent".
Any hints or ideas are really appreciated.

Thank you in advance, lg Clemens