DDSXTY14 — Must Undestand rules are not working well with appendable types

Checking assignability from T2 to T1 for appendable types where T2 has some extra members that are marked optional and must_understand cannot be implemented the way it is specified.

Assume:

@appendable
struct ReaderType {
   int32 m1;
};

@appendable
struct WriterType {
   int32 m1;
   @optional @must_understand
    int32 m_opt_must_understand;
};

According to assignability rules these are compatible but the reader needs to drop the WriterType samples that contain the m_opt_must_understand field set (not all samples since it is optional).

However given how XCDR de-serializes appendable types it will never look to see if those extra fields are set since they are not part of T1 and the T1 deserializaton will skip everything after deserializing m1 using the DHEADER.