Shotcut: v21.09.20 Release

This main purpose of this version is to fix a major new bug in previous version 21.08.

• Fixed x86-64 CPU compatibility in Linux and macOS releases (broke in v21.09.13).
• Fixed dragging to Timeline broken if there is MLT XML on the clipboard (broke v21.08.29).
• Fixed seeking backwards after seeking to the end (broke in v21.08.29).
• Fixed File > Save (Ctrl-S) immediately after File > Close (Ctrl-W) can corrupt a saved project.
• Fixed Add Custom Video Mode > Frames/sec was not accepting some legitimate values.
• Fixed encoding HEVC without hardware encoder (x265) would always output 10-bit instead of 8 (broke in v20.11.28).
• Changed Copy the filters to only copy enabled filters.
• Changed fading on the Timeline to automatically fade the opacity when it is not the bottom video track.
• Added direct support for GoPro .LRV files as proxy videos. These must be in the same folder as the corresponding MP4 file and proxy mode enabled.
• Added 360: Equirectangular to Stereographic video filter. This can be used to create so-called Tiny Planet videos.
• Expire old QML cache items at startup to reduce app data footprint.
• Improved performance of 360: video filters.