[cairo] fixed_from_double speedup masking pixman traps problem

[Bill Spitzak]

In the fast fixed-from-double code, overflow can be detected because the 
"high word" of the resulting double after the math must be a constant. 
If it is not the expected value then overflow occurred and you can then 
use some slower code to do the conversion.

I tried this here but the overhead of the if statement killed most of 
the speed advantage, also investigation of our code indicated that at 
least 90% or so of all calls to our float->int converter was from code 
that already knew the number was in range. So I did not do this. However 
it does appear to be the fastest way to make a flawless converter.

Bogdan Nicula wrote:
> 
> I mentioned before that it's easy to overflow in this function 
> (fixed_from_double) due to errors in  other code. Is it possible to add 
> some input checking?
> 
> Bogdan
> 
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