slices. With this new code libmpeg2's behaviour should be equivalent
to that of decoders that have the notion of macroblock addresses. The
speed impact should not be measurable as this is outside of the
macroblock loop.

Also checking that the macroblock position is within the bounds of the
image probably helps with robustness.

putenv is posix, setenv is a bsdism

dont overflow as much as the reference decoder does :)

updated test suite accordingly: -c is always used unless we explicitly
specify an idct other than c. That way the regression test should pass
on all architectures.