The intrax8 decoding process does not imply any kind of error
resilience, and the only call present is more related to how mpegvideo
works rather than anything else.
Therefore have the parent decoders carry out er when actually needed.
Keep the code as similar as possible across the codepaths to
ease spotting it for factorization.
Based on a patch from Michael Niedermayer <michaelni@gmx.at>.
The code currently set the information in at least 4 places, spare
some pointless loops.
Make the code in the loop a little uniform to make easier factorize
it out later.
