Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/mlt/versions.

Version release notes (from GitHub):
This version fixes bugs and supports serializing animation keyframes with a
specified time format (previously only frame number).

**Framework**

  * Reverted mlt_pool change in v6.8.0 pending further testing.
    (USE_MLT_POOL compiler define is now a 0/1 boolean, defaults to 1.)
  * Fixed crash regression in v6.8.0 "parsing non-animated string as an animation."
  * Added pointer checks to mlt_animation.
  * Changed producer cache size heuristic in mlt_multitrack to be more liberal.
  * Fixed handling reserved characters in names for YAML in mlt_properties.
  * Added clamping to prevent computing negative in and out points to mlt_producer.
  * Added functions to serialize animation with a time format:
    - mlt_animation_serialize_cut_tf()
    - mlt_animation_serialize_tf()
    - mlt_property_get_string_tf()
    - mlt_property_get_string_l_tf()
    - mlt_properties_get_value_tf()
    - Mlt::Properties::get(int, mlt_time_format)
    - Mlt::Animation::serialize_cut(mlt_time_format, int, int)
  * Added functions to clear a property to mlt_properties:
    - mlt_property_clear()
    - mlt_properties_clear()
    - Mlt::Properties::clear()

**Modules**

  * Fixed enabling sliced pix_fmt conversion in avformat producer.
  * Fixed incorrect seek and sync on audio files with discard packets.
  * Added support for avcodec_send_frame() API to avformat consumer.
  * Fixed compile errors with Libav master.
  * Fixed a crash in affine transition.
  * Fixed a crash in ladspa filters when consumer frame rate is low (e.g. <= 8).
  * Fixed a crash in boxblur filter.
  * Added animation support to boxblur hori and vert properties.
  * Fixed a crash in movit.convert.
  * Fixed incorrect alpha in affine transition blending routine.
  * Converted frei0r from deprecated mlt_geometry to mlt_animation API.
  * Fixed tilde in text string for pango producer.
  * Fixed using more than one channelcopy filter.
  * Fixed the mono filter reducing volume level.
  * Fixed degraded audio scrubbing in sdl2_audio consumer.
  * Converted dynamictext filter to use affine transition for more correct
    alpha compositing and sub-pixel positioning.
  * Added time format support for animation keyframes to the xml consumer.
  * Added animation support to more affine transition properties:
    - fix_rotate_x
    - fix_rotate_y
    - fix_rotate_z
    - fix_shear_x
    - fix_shear_y
    - fix_shear_z
    - ox
    - oy
    - scale_x
    - scale_y
  * Fixed gaps in text when characters overlap in qtext and kdenlive producers.
  * Fixed a crash in pixbuf producer with multiple render threads.
  * Converted the oldfilm vignette filter from mlt_geometry to mlt_animation.

**Other**

  * Numerous updates to mlt-xml.dtd.
  * Categorized many of the encode presets (using meta.preset.name).

These checks were done:

- built on NixOS
- /nix/store/jfzwhslr6ndh2wc4bl3vdm4kdk2xdk7a-mlt-6.10.0/bin/melt passed the binary check.
- /nix/store/jfzwhslr6ndh2wc4bl3vdm4kdk2xdk7a-mlt-6.10.0/bin/.melt-wrapped passed the binary check.
- 2 of 2 passed binary check by having a zero exit code.
- 0 of 2 passed binary check by having the new version present in output.
- found 6.10.0 with grep in /nix/store/jfzwhslr6ndh2wc4bl3vdm4kdk2xdk7a-mlt-6.10.0
- directory tree listing: https://gist.github.com/5b163c9c1aa200eba8db3bde8c8b09b1
- du listing: https://gist.github.com/ddb0aac5012b87f121601378fca1f0bd