[cairo] API proposal: Damage Tracking

[Bill Spitzak]

For hit detection I think it would be better to make a hit-detecting 
Cairo surface type, rather than adding any calls to all existing 
surfaces. You create such a surface, draw to it, then use 
surface-specific calls to find out what was hit.

In my experience nobody cares about the alpha of what was drawn. They 
only want to know if the path or stroke hit a specific area, even if 
what was drawn was a transparent image. Programs are far more interested 
in the source x/y of a copied image that was hit, they can then (if they 
want) look at the image to figure out if it was a transparent pixel.