compress: Avoid PARA_ABS and PARA_MAX in inner loop.

These macros are type-safe and evaluate their arguments only once.
However, they are also slow. For the compress filter we are dealing
with ints and unsigned ints only, so the additional checks performed
by the macros only slow things down. Getting rid of the macros reduces
the running time of

	para_filter -f compress < foo.wav > /dev/null

by approximately 8%.