WIP factoring out MovieBox sample adjustment logic from mdat offset calculations/adjustments

Put the MovieBox-specific logic for IsoFileContainerBox.trim_to_time_range ()
 into IsoMovieBox.remove_samples_outside_range() and
 adjust_track_chunk_offsets(). This will allow the logic to be re-used for
 the fragmented sample adjustment logic.

Un-did some of the premature factoring of IsoFileContainerBox trim logic
 into trim_fragmented/trim_non_fragmented. There's too much logic overlap
 for separation at that level.

Confirmed that recent timerange test cases are still producing valid mp4s.