Fixed a bug in IsoFileContainerBox.trim_to_time_range() dealing with time ranges that start in the MovieBox and end in a MovieFragmentBox

Moved the logic for calculating the MovieBox size change outside of the
 start point trim logic. The size change wasn't being dealt with
 if/when the start point was at the very start of the MovieBox.

Changed the MovieBox mdat reference logic. Removed IsoFileContainer.
 get_mdat_for_offset() and replaced it with IsoMovieBox.get_referenced_mdats().

Tested with various scenarios of B-AVC_MP4_MP_SD-01.MP4.