multimedia/shotcut: update to 21.03.21

From ChangeLog: https://shotcut.org/blog/new-release-210321/

• Fixed <tractor> and blank considered missing and blocking export
• Added an Ignore button to the missing files error dialog in Export.
• Fixed black frames appearing in Slideshow Generator
• Fixed being able to use mouse wheel over Timeline and Keyframes zoom sliders
• Improved sound quality with small changes in Speed with Pitch Compensation or Pitch audio filter (broke in v20.11).
• Fixed the Timeline clip audio waveform incorrect after adding transition by dragging the clip left.
• Fixed repairing a project file opened using a network path on Windows.
• Fixed writing a raw FLAC file does not set its duration.
• Fixed an elided file name in clip Properties can become the clip name.

Deleteing patch integrated upstream: https://github.com/mltframework/shotcut/pull/1050

Reported by: portscout