Add a enable/disable switch for using segmantation

By default, segmentation is turned off.
Note that there is an existing similar flag 'segmentation.enabled' under
SegmentationState struct under TileState, and the new one from this commit is
the master switch and later will be controllable by CLI.

By not using any segment, this temporarilly fixes the ongoing desync issues caused by quantizer rdo and
heuristic quantizer choice at speed 0,1,2.
12 jobs for pr-1923/ycho/rav1e/add_enable_segmentation_switch_in_FI in 31 minutes and 26 seconds (queued for 86 minutes and 1 second)
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CAPI and Debugging Features

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passed #9963
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passed #9959
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Latest Stable Rustc

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passed #9960
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Minimum Rustc

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passed #9961
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00:02:57

 
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passed #9965
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Code Styles

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skipped #9966
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Build rav1e
 
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Slow Tests (aom)
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Slow Tests (dav1d)
 
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Generate Docs
 
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= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unknown_clippy_lints

error: aborting due to 2 previous errors

error: Could not compile `rav1e`.

To learn more, run the command again with --verbose.
ERROR: Job failed: exit code 1