X/Render, rotation transform, Src PICTOP: can I ignore pixels in the bounding box but outside the polygon?

[Nigel Tao]

On Wed, May 18, 2016 at 8:07 PM, Ilya Anfimov <ilan at astelecom.ru> wrote:
>  But  anyway  you can specify clip-mask with ChangePicture to use
> bitmap of enabled pixels. Whould be better IMO  than  restricting
> output line-by-line.
>  And that mask bitmap also could be calculated by RENDER.

I suppose that will 'work', clipping the dst rather than the src,
although it seems inefficient to allocate and clean up an entire
Pixmap for the mask every time I want to do this...