[cairo] clear operator

[control H]

> Using http://cairographics.org/operators/ as reference:
>  - paint implies a mask which covers the whole surface (1 everywhere)
>  - CLEAR ignores the context source
>  - you have not clipped to anything
>
> The expected result can be computed based on the "Bounded" formula:
> (src CLEAR dst) LERP_(clip IN mask) dst =
> (0) LERP_(1 IN 1) dst =
> 0 LERP_1 dst = dst

OK so it is because in my case something is not clipped or bounded.
But why does the first example (CLEAR) on
http://cairographics.org/operators/ give the result I expect? Why is
something clipped there? I looked at the code but I can't figure out.