Move the size information in EbmlCallbacks

[robUx4]

SizeIsValid() is used before reading the data. It's only used to check the header if the size is valid. So we could move the check in the EbmlCallbacks (holding the specs of each element).

Eventually we can also avoid allocating an element if we know beforehand the size is not valid.

[robUx4]

The problem is that EbmlCallbacks are constexpr classes, and we can't have virtual methods in there...

[robUx4]

It is possible with an old school C callback type. We already use one for the Creator.