[PATCH] use magic wl_fixed_t to/from double only on x86-64

[Bill Spitzak]

Thiago Macieira wrote:

> However, the div and mul are much more interesting. Those ought to be the 
> same, so I am actually wondering how it is possible that the div and mul on 
> x86-64 can be so slow.

Divide by x and multiply by 1/x are not exactly the same because 1/x may 
not be exactly represented in the floating point format. So I think this 
is the reason the compiler is not producing the optimization. Though 
1/256 can be represented exactly so it does not apply to this specific case.