PR #2940 and #2973 added an offset field to drmodtrack for #2939 (closed), but they ifdef-ed the field in some places but not others, resulting in uninitialized output. Since it's already locked into the interface, we always set it to 0 here and include it in internal structures to fix the problem.
Tested on drmodtrack-test where the fields were manually confirmed to no longer contain bogus values, and where the test doesn't fail when i#4474 adds a new field that shifts the test buffer to avoid the happens-to-match scenario that masked this bug in that test before.
Issue: #2939 (closed), #4474, #4777 (closed) Fixes #4777 (closed)