llvmpipe: use ints not unsigned for slots

llvmpipe: use ints not unsigned for slots

They can't actually be 0 (as position is there) but should avoid confusion.

This was supposed to have been done by af7ba989fb5a39925a0a1261ed281fe7f48a16cf
but I accidentally pushed an older version of the patch in the end...
Also prettify slightly. And make some notes about the confusing and useless
fs input "map".

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>