Cached reading of stsz atom

The entire stsz atom needs to be checked, entry by entry, to
determine the maximum size of a sample so that the proper buffer
size can be allocated by the decoder. This change reads a block of
entries at a time, cutting down the number of file reads and thus
significantly increasing the performance of reading the entries.

Change-Id: Ida39c2fe59bccb4440faeab1e71d8cf58682bcc3