MKVToolNix 57.0.0 (64-bit)

What's new in this version:

New features and enhancements:
- mkvmerge: MP4 reader: added support for reading Dolby Vision from MP4 files (FourCCs `dvh1` and `dvhe`; configuration records `dvcC`, `dvvC` and `hvcE` will be converted into block addition mappings).
- mkvmerge: SRT subtitles: mkvmerge now accepts empty text files with the extension `.srt` as SRT subtitle files, enabling the creation of empty SRT tracks
- mkvmerge, mkvextract: HEVC/H.265: access unit delimiter NALUs will no longer be discarded, neither during muxing nor during extraction
- MKVToolNix GUI: preferences: switched the order & wording of controls in the "enabling items" panel to make it clearer that certain controls define exceptions

Fixed:
- mkvmerge: HEVC/H.265 parser: several NALU types, notably the Dolby Vision-specific NALUs ("unspecified 62" and "unspecified 63") and suffix SEI NALUs, are now stored with the frame they belong to instead of with the next frame.
- mkvmerge: HEVC/H.265 packetizer: fixed setting the track's default duration when reading HEVC/H.265 from Matroska files that don't have a default duration set.
- mkvmerge: HEVC/H.265 packetizer: fixed the calculation of the duration of frames so that `SimpleBlock` elements can be used again instead of `BlockGroups` with `BlockDuration` elements
- mkvmerge, mkvextract: HEVC/H.265 parser: fixed issues with ordering & duplication of certain NALUs (parameter set & prefix SEI NALUs)
- MKVToolNix GUI: multiplexer: when dragging & dropping files to the multiplexer, the source directory will be remembered as the "last open directory" again, causing subsequent uses of the "open file" dialog to start in the same directory.
- mkvinfo: fixed the position of frames in block groups containing "block duration" elements in summary mode (e.g. for subtitle tracks)

Build system changes:
- `configure` will now try to detect `libmagic` via `pkg-config` and fall back to including & linking directly if it cannot be found via `pkg-config`