[cairo] render in grayscale

[Per Hermansson]

Soeren Sandmann wrote:
> In cairo master, the HSL_LUMINOSITY operator can be used to get a
> grayscale conversion: compositing the RGB24 on top of opaque white
> using this operator will generate a properly converted grayscale
> image. The attached program will demonstrate this if you set
> "algorithm" to USE_LUMINOSITY.
>
> If you run it as is, it will just show the color drawing.
>
>
> Soren
>   
Thanks for the help, this seems like the best approach to use when the 
next cairo version is released.
Until then a possible solution might to pre-process the pdf before 
rendering it.

Per