[Pixman] Use of bitwise operators on vectors with Boolean results

[Michael Tautschnig]

[...]
> > as, at least on the platforms that I tried, comparisons on vectors yield
> > (unsigned)-1 as representation of "true."
> 
> Yes, this behaves exactly as documented at:
> 
>     https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html
> 
> "Vectors are compared element-wise producing 0 when comparison is false
> and -1 (constant of the appropriate type where all bits are set)
> otherwise."
> 

Thank you very much for the elaboration!

Sorry for the noise,
Michael