[whatwg] Canvas: clarification of compositing operations needed

[David Flanagan]

Firefox and Chrome disagree about the implementation of the 
destination-atop, source-in, destination-in, and source-out compositing 
operators.  Test code is attached.

Chrome doesn't touch any destination pixels that are not underneath the 
source pixels.  Firefox, on the other hand, treats the entire canvas 
(inside the clipping region) as the destination and if you use the 
destination-in operator, for example, will erase any pixels outside of 
whatever you are drawing.

I suspect, based on the reference to an "infinite transparent black 
bitmap" in 4.8.11.1.13 Drawing model that Firefox gets this right and 
Chrome gets it wrong, but it would be nice to have that confirmed.

I suggest clarifying 4.8.11.1.3 Compositing to mention that the 
compositing operation takes place on all pixels within the clipping 
region, and that some compositing operators clear large portions of the 
canvas.

        David