Add SegmentationLevel::Complex and set for speed 0

[barrbrain]

AWCY result at speed 0	PSNR Y	PSNR Cb	PSNR Cr	CIEDE2000	SSIM	MS-SSIM	PSNR-HVS Y	PSNR-HVS Cb	PSNR-HVS Cr	PSNR-HVS	VMAF	VMAF-NEG
1.0826	-0.2250	-0.0412	0.2315	0.0593	0.1193	0.8248	-0.8888	-0.6546	0.7733	1.0963	1.1068

Average encoding speed-up of 1.41x, with some clips averaging 2.16x. In the most extreme case, 2.63x for ducks_take_off at Q=80.

Note that that AWCY results relative to speed 1 show a healthier 1.26x encoding time for most of the BD-Rate improvement:	PSNR Y	PSNR Cb	PSNR Cr	CIEDE2000	SSIM	MS-SSIM	PSNR-HVS Y	PSNR-HVS Cb	PSNR-HVS Cr	PSNR-HVS	VMAF	VMAF-NEG
-2.6694	1.4971	0.8145	-0.9577	-1.1083	-0.8471	-2.2635	2.7948	1.5700	-2.1606	-2.7774	-2.6214

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f3c8d7b) 88.49% compared to head (5e09941) 88.48%.

Files	Patch %	Lines
src/api/config/speedsettings.rs	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3319 +/- ## ========================================== - Coverage 88.49% 88.48% -0.01% ========================================== Files 88 88 Lines 28235 28236 +1 ========================================== Hits 24986 24986 - Misses 3249 3250 +1 ```

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[lu-zero]

It looks very promising