MyISAM/Aria: Adding/removing non-materialized virtual column triggers table recreation

This is a split from MDEV-6103:
MyISAM (and most probably Aria) reserves extra bits for virtual columns (NULL flag) in a physical record (the one that is written to disk). There should be no good reason to do so.

E.g.:
create table t1 (b int, d int); # Uses 2nd and 3rd bits for NULL flags
create table t2 (a int as (-b), b int, c int as (-b), d int); # Uses 3rd and 5th bits for NULL flags

All in all it means physical record format changes when adding/dropping virtual columns and just relaxing table comparison function is not enough to fix this bug.

See test case for MDEV-6103 in vcol.vcol_non_stored_columns_myisam which confirms reported behavior.