[postprocessor:ugoira] use 'image2' demuxer (#1550)

Ensures exact frame timecodes with no duplicate frames.

Possible issues are the duration the last frame in an Ugoira with variable
frame durations is shown and insufficient timestamp precision of the
underlying file system (e.g. FAT32, ext3; works on ext4, tmpfs, NTFS).

docs/configuration.rst

@@ -2498,6 +2498,18 @@ Description
     Additional FFmpeg command-line arguments.
 
 
+ugoira.ffmpeg-demuxer
+---------------------
+Type
+    ``string``
+Default
+    ``image2``
+Description
+    FFmpeg demuxer to read input files with. Possible values are
+    "`image2 <https://ffmpeg.org/ffmpeg-formats.html#image2-1>`_" and
+    "`concat <https://ffmpeg.org/ffmpeg-formats.html#concat-1>`_".
+
+
 ugoira.ffmpeg-location
 ----------------------
 Type