Fixed a bug with end trimming

IsoSampleToChunkBox.remove_sample_refs_after() needed to have different
 logic for one-chunk entries vs multi-chunk entries. If the sample is
 part of a single-chunk entry, then just the samples_per_chunk needs to
 be modified on the existing entry. For a multi-chunk entry, the next
 entry needs to be modified to account for truncated chunks.

Tested with various closed time ranges with both O-AVCMP4_BC15A-02.MP4 and
 B-AVC_MP4_MP_SD-03.MP4. All tested time ranges work as expected.