avformat/matroskaenc: Check chapter ids for duplicates

Up until now, there has been no check that each chapter has a unique id;
there was only a check for whether a chapter id is zero (this happens
often when the chapters originated from a format that lacks the concept
of chapter id and simply counts from zero) which is invalid in Matroska.
In this case the chapter ids are offset by 1 to make them nonnegative.
Yet offsetting won't fix duplicate ids, therefore this is changed to
simply create new chapter uids when the input chapter uids don't conform
to the requirements of Matroska (in which case it can be presumed that
they did not originate from Matroska, so that we don't need to bother
to preserve them).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

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