Perlin noise benchmark speed

[bearophile via Digitalmars-d]

dennis luehring:

> it does not makes sense to "optmized" this example more and 
> more - it should be fast with the original version

But the original code is not fast. So someone has to find what's 
broken. I have shown part of the broken parts to fix (floor on 
ldc2).

Also, the original code is not written in a fully idiomatic way, 
also because unfortunately today the "lazy" way to write D code 
is not always the best/right way (example: you have to add ton of 
immutable/const, and annotations, because immutability is not the 
default), so a code fix is good.

Bye,
bearophile