[cairo] A8 images as source?

[Bill Spitzak]

Doing cairo_set_source_surface(cr, anA8image) and drawing reveals that 
it treats the A8 cairo_image_t as having r=g=b=0, and only the alpha 
channel non-zero.

I'm wondering if instead it should instead act as though r=g=b=a. Two 
reasons:

- Using CAIRO_OPERATOR_SOURCE and the A8 image can be used to draw a 
grayscale image. This is probably not a big deal but it was what I was 
trying to do.

- Treating the A8 image this way is probably necessary if colored masks 
are ever supported.

This would change the results whenever an A8 image is used as the source 
(rather than for cairo_mask_surface()). For instance a normal over would 
draw white instead of black (but you could use DEST_OUT or some other 
operators to get black again). So I am not sure how many programs this 
would break. An alternative is to add yet another image format.