So we have access to the semantic constraints in the constructor.

Only master elements can and only very few.

And assert if an illegal call is done.

Only master elements can be.

Because it's a binary element, not a Master element.

Setting whatever on an EbmlMaster returns true.
Setting true on other classes returns false.
Setting false on other classes returns true.

Fixes an issue in EbmlElement::FindNextID() where an EbmlMaster was set to
infinite/unknown even though it's not allowed for that element.

EbmlElement::FindNextElement() would also allow an infinite/unknown size
for any master element even though it's not allowed.

That should improve error recovery by skipping more invalid sizes given the reading context.

We can simplify use the semantic of each element directly.

And the assert will work all any element, not just the master ones.

It is called in FindNextId/FindNextElement which always checks for failure.
So don't need the assert there.

It's also called from the EbmlMaster constructor.
The copy constructor is fine since the the original can only have a valid state.
The other constructor has a default value of finite size and the macros never set this to infinite.
In libmatroska the KaxSegment constructor calls SetSizeInfinite() rather than using the default value.
But we don't need an assert there, since we know it's allowed.