[cairo] fixed_from_double speedup masking pixman traps problem

[chris nuernberger]

The sledgehammer would help see the problem quickly should it ever
arise again.  I may be mistaken but this seems like a circumstance
that:

a. produces output that looks to the rest of the world like correct output and
b. could easily produce subtle errors taking a lot of time to find.

I agree to fix pixman, that doesn't seem arguable at all.  But neither
does some insurance against this happening again or in other places.

Chris