allow chapter duration of 0

There can be segments with just chapters containing commands.

In ordered chapters the content will not play anything, it may seek to the
position for nothing but that's not a big deal.

Fixes #542

chapters.md

@@ -110,13 +110,13 @@ The timestamp of the end of `Chapter` with nanosecond accuracy, not scaled by Ti
 The timestamp defined by the `ChapterTimeEnd` is not part of the `Chapter`.
 A `Matroska Player` calculates the duration of this `Chapter` using the difference between the
 `ChapterTimeEnd` and `ChapterTimeStart`.
-The end timestamp **MUST** be strictly greater than the start timestamp.
+The end timestamp **MUST** be greater or equal than the start timestamp.
 
 Chapter   | Start timestamp | End timestamp | Duration
 :---------|:----------------|:--------------|:-----
 Chapter 1 | 0               | 1000000000    | 1000000000
 Chapter 2 | 1000000000      | 5000000000    | 4000000000
-Chapter 3 | 6000000000      | 6000000000    | Invalid (0)
+Chapter 3 | 6000000000      | 6000000000    | 0
 Chapter 4 | 9000000000      | 8000000000    | Invalid (-1000000000)
 Table: ChapterTimeEnd usage possibilities{#ChapterTimeEndUsage}
 

ebml_matroska.xml

@@ -1160,7 +1160,7 @@ Use for WebVTT cue identifier storage [@!WebVTT].</documentation>
   </element>
   <element name="ChapterTimeEnd" path="\Segment\Chapters\EditionEntry\+ChapterAtom\ChapterTimeEnd" id="0x92" type="uinteger" maxOccurs="1">
     <documentation lang="en" purpose="definition">Timestamp of the end of Chapter (timestamp excluded, not scaled).
-The value **MUST** be strictly greater than the `ChapterTimeStart` of the same `ChapterAtom`.</documentation>
+The value **MUST** be greater or equal the `ChapterTimeStart` of the same `ChapterAtom`.</documentation>
     <documentation lang="en" purpose="usage notes">If the Edition is an ordered edition, see (#editionflagordered),  then this Element is **REQUIRED**.</documentation>
     <extension type="webmproject.org" webm="1"/>
   </element>