[cairo] clear operator

[control H]

> In the example, the CLEAR operator is used when filling the "blue rectangle"
> with an opaque blue solid color.
> You are using paint(), so you're doing the same operation as that
> example, but with a rectangle that covers the whole surface.

That's CLEAR :) Thanks for explaining.
Part of my confusion though was because I presumed transparent
background does not do anything, since on the website it was phrased:

Where the second object is drawn, the first is completely removed.

In my interpretation that means that whenever a pixel is transparent,
it will not be drawn on the second object and therefore not removed.
Perhaps this might confuse other people as well and the documentation
can make a remark about it.

Anyway, thanks for your answers Andrea.
regards.