Small refactors of pixels ROD decoder

This MR is just a technical refactor aiming to make the decoding process more understandable (shorter). Tagging @stsuno @goetz

Background. I was progressing with making IDC BS container used in PixelErrorsTool but realised some info is missing. Needs fixing in this complicated decoder code. So, after two days of looking into it, I thought it could profit from some small modernisations basically making the main loop shorter. Scope of variables is shortened thus reducing possible mistake of reuse of old value from the previous iteration. Results are not expected to change.