[cairo] fixed_from_double speedup masking pixman traps problem

[chris nuernberger]

I would vote for two things.

The first is what Daniel is proposing for sure.  The sooner you catch
the problem the more descriptive the error is going to be.

The second is to put checks in in debug mode only in the function, or
to have some macro switch to turn them on or off.

Ideally cairo could have a pedantic mode where it checked everything
and you could turn this on or off.  It may save bug reports in the
future, and my experience with graphics in general is that it is
complex enough that you save considerable time down the road
implementing at least some system that helps users pinpoint their
errors unless you want to spend every waking moment helping people
figure out why their program is always displaying something odd
(thanks opengl).


--the guy who inspired Joonas