really allow 0 duration chapters

[robUx4]

In #464 I said it was not necessary. In retrospect I think a duration of 0 for some chapters is necessary.

In ordered editions, as said above, you can merge things you need for that chapter in neighbor chapters. Only a marker right at the end of the file may not be possible but I'm not sure it's of any use.

I found a simple DMX generated file (from 2005) which has a duration of 0:

<Chapters>
 <EditionEntry>
  <EditionUID>1676494232</EditionUID>
  <EditionFlagOrdered>1</EditionFlagOrdered>
  <!-- First Play PGC -->
  <ChapterAtom>
   <ChapterUID>1246888591</ChapterUID>
   <ChapterTimeStart>00:00:00.000000000</ChapterTimeStart>
   <ChapterTimeEnd>00:00:00.000000000</ChapterTimeEnd>

   <ChapterDisplay>
    <ChapterString>First Play PGC 1246888591</ChapterString>
   </ChapterDisplay>

   <ChapterProcess>
    <ChapterProcessCodecID>1</ChapterProcessCodecID>
    <ChapterProcessPrivate format="hex">30 00 00 00 </ChapterProcessPrivate>
    <!-- Pre commands -->
    <ChapterProcessCommand>
     <ChapterProcessTime>1</ChapterProcessTime>
     <ChapterProcessData format="hex">01 30 02 00 00 00 02 00 00 </ChapterProcessData>
    </ChapterProcessCommand>
   </ChapterProcess>
  </ChapterAtom>
 </EditionEntry>
</Chapters>

This is actually the equivalent of the entry point of the DVD (first play program chain). And because it uses DVD commands (when entering) it cannot be merged into another chapter that would also start at 00:00:00.000000000, because it may also have some commands on entering. (I do have this case as well)

One reason not to allow 0 duration was the odd situation where the ChapterTimeEnd value is excluded from the chapter content to play (in ordered editions). And at the same time it's included in ChapterTimeStart. But we can just say that in this case the duration is 0. The player should just enter/leave such a chapter nonetheless during playback, in order to handle the ChapProcess content.

[hubblec4]

Yeah, I have ever prefered such a 0-duration for ordered chapters.
I have also seen such chapter.xml files from DMX.