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 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.
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