By having a layer as a short (16 bits) we can pack it together with
the bitfields, saving 4 bytes per sub-struct, 8 bytes in total, also
bringing the struct down from 4 to 3 cachelines on my laptop.
Rationale: layers are mostly used to differentiate groups of objects
and they stacking, usually we have few layers and we use very large or
very small numbers to make a layer be at the top or at the bottom, but
usually we don't need so many layers.
Caution: code that use values like 999999 will break, so fix your
code! I'll provide another patch to fix all the CVS using these large
values.
SVN revision: 34420
27437c83cf