M4A: Support 64 bit atom sizes

[jmcintyre]

When a 64bit atom size is detected, the larger size will not be retrieved after header.

[mathiascode]

Thanks! This needs a sample file in https://github.com/tinytag/tinytag/blob/master/tinytag/tests/test_all.py. You can truncate an existing file to only include the header, and add it to https://github.com/tinytag/tinytag/tree/master/tinytag/tests/samples.

In order to keep the correct atom size in the debug message, I would add a data_offset variable (or similar) that contains the position of the data after the header. Once you've logged atom_size, you can subtract data_offset from it, and possibly store the result in a new variable.