[cairo] IncludeInferiors, Pixmap/Window distinction

[Bill Spitzak]

Karl Tomlinson wrote:

>> I think you should get an image that is the
>> size of the window (ie not clipped to the screen area), and that
>> "wrong" pixels should be transparent black, not unknown
>> values.
> 
> Hmm. Sounds good, but also tricky with IncludeInferiors.

What I meant was that the back end can decide whether inferior windows 
are included. If it can't do it then it should produce transparent 
black, not some undefined result. If it can produce them it should, 
since users would prefer that.