Weekly pre-release

What's Changed

• Improved scene detection by @master-of-zen in #2710

Current fast scene detection in rav1e relatively slow to compared methods of scene detection, and have proclivity to show false results or don't detect scene changes where they appear.

This PR reworks fast scene detection algorithm, making it faster, better, and more accurate.

Achieved goals are:

• Faster decision making ( Both less and more efficient computations )
• More accurate scene detection, by adjusting threshold based on previous frames
• Frame downscale for faster decisions

Full Changelog: p20210713...p20210720