draw,llvmpipe: use exponent manipulation instead of exp2 for polygon offset

draw,llvmpipe: use exponent manipulation instead of exp2 for polygon offset

Since we explicitly require a integer input we should avoid using exp2 math
(even if we were using optimized versions), which turns the exp2 into a int
sub (plus some casts).

v2: fix bogus uint (needs to be int) math spotted by Matthew, fix comments

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>