Fix #8589, 653e7fa548: Motion counter is NewGRF-visible but not saved, leading to desyncs. #8591
Motivation / Problem
Vehicle motion counter is NewGRF-accessible, but not stored in saves. This can lead to desyncs, see #8589.
Store motion counter in saves to make access desync safe. Motion counter was not touched since its initial r6532.
Checklist for review
Some things are not automated, and forgotten often. This list is a reminder for the reviewers.