Especially when dealing with big files the performance is quite bad. This
change speeds it up a little by using an unordered map to find the elements
for certain offsets instead of using linear lookup.
According to callgrind the modified functions where one with the biggest
own cost. The function updateRelativeOffsets() was actually the 2nd costly
function just below __memcpy_avx_unaligned_erms.
Now TagParser::EbmlElement::internalParse(TagParser::Diagnostics&) is quite
high as well as standard IO stream functions.
This should fix all non-erros, leaving only warnings which
are indeed potential problems.
The following warnings should be safe to ignore:
* Conversions of various offsets from uint64 to
std::streamoff/int64 are safe because such offsets have
been obtained via tellg() and other functions
returning std::streamoff in the first place.
* It also works vice-versa since tellg() should not
return negative offsets with exceptions enabled.
* Conversions from char to unsigned char are also ok.
* Unused diag arguments can be ignored (those might be
useful later).
* Annotate all intended fallthoughs.
Martchus