[cairo] cairo_set_operator() sanity check

[Doodle]

Hi,

I suddenly experienced a problem with my Cairo testapp, which selects 
random operators and draws random shapes on the screen.

Now, I made a mistake in there, which resulted in that 
cairo_set_operator() was sometimes called with operator number 14, which 
of course does not exist. The testapp crashed at cairo_stroke() at once.

I know, it's user error, but I'm interested in your opinions, shouldn't 
such trivial checks be in Cairo functions (well, up to a limit, of course)?


Doodle