matroska: add readme files for matroska data
matroska: add a script to create images for testing
matroska: create base test videos
matroska: generate videos for testing
matroska: commit generated video and chapter files
matroska: make groups of videos
matroska: create test videos for misaligned chapter references
matroska: add code for generating chapter references with gaps
matroska: ignore png and txt files generated by the script
matroska: commit videos for other test cases