qtmux: handle overlapping subtitle buffers properly

Submitted by Matej @Knopp

Link to original bug (#741306)

Description

Overlapping subtitle buffers (buffers that start before previous buffer ends) are legal in many containers (i.e. MKV), but can not be represented in MP4. Trying to remux such file currently results in invalid file, because timestamp delta between buffer end time and next buffer start time is negative

In order to handle this well, the muxer should split and combine buffers in a way so that no information is lost. I.e. Two overlapping input buffers should be muxed as three separate buffers, with second buffer combining content from both input buffers.

[=======Buffer In 1========]
[=======Buffer In 2========]

[Buffer Out 1][Buffer Out 2][Buffer Out 3]

Sample MKV file with overlapped subtitles
https://s3.amazonaws.com/MatejK/Samples/OverlappedSubtitles.mkv

Remuxed file without the patch
https://s3.amazonaws.com/MatejK/Samples/OverlappedSubtitlesWithoutPatch.mp4

Remuxed file with the patch
https://s3.amazonaws.com/MatejK/Samples/OverlappedSubtitlesWithPatch.mp4