Design limitation of read boundaries

[robUx4]

We recently got a file that has some upper element that violates its size boundaries.

When calling EbmlElement::FindNextElement() one can pass the limit to which the elements can be found with MaxDataSize. However that value is not used if we read the next upper element. There's a check on the value but if it's a binary or master element with no constraints it will be valid no matter what, and returned because UpperLevel > 0.

[robUx4]

To avoid avoid considering this element with a bogus size as valid in this context, we could either pass the size constraints for each level. We could also use a callback that passes the context and the element and ask the user if this element is OK. This latter suggestion could also replace the AllowDummyElt flag as the host could decide that dummies are OK in some places but not others.

[robUx4]

A callback where the caller can validate elements would also allow checking that the CRC32 is the first element in its parent (or within a parent), otherwise it's not in the correct place and likely a false positive read, given it's EBML ID is 0xBF.