[cairo] a couple requests from Inkscape

[Behdad Esfahbod]

Carl Worth wrote:
>> - inversion operator, i.e. rendering an object by inverting what is
>> below it in z-order. With the existing operators it's impossible,
>> although it is no less (and probably more) useful than e.g. xor. In
>> SVG, inversion can be done by a filter; in other vector backends, it
>> will need rasterization (but you use rasterization anyway for other
>> operators).

Isn't this conceptually possible by painting using an alpha of -1 to a white
surface?  That may be in our grand plan for when we support HDR and color
values beyond [0..1].

behdad