Mp4: remove useless leading stsc entry in result mp4. The fix removes useless stsc entry in result mp4. If start_sample == n then current stsc entry should be skipped and the result stsc should start with the next entry. The reason for that is start_sample starts from 0, not 1.

# Copyright (C) Igor Sysoev
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have=NGX_SUPPRESS_WARN . auto/have

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