[cairo] fixed_from_double speedup masking pixman traps problem

[Bogdan Nicula]

>I should have mentioned that the old version of fixed_from_double
>overflowed just as 'easily' as the new one does. The difference is
>that the result of the overflow is different, which what Vlad meant by
>"we're getting different bogus values."

Of course I knew this.

May be it still is preferable to stop on overflow here rather than 
continuing with "bogus values", potentially corrupting memory. Remember, 
cairo is critical infrastructure.

Bogdan

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