Render regions in fibers

This reverts commit cf8d49adbb36640bf98278ed7613d7d1a2e29060.

Closes #3496835

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• dc3f27f5 - Render regions in fibers.

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• e47da374 - Render regions in fibers.

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added 1 commit

• a6036ef0 - Add iterations support etc.

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added 2 commits

• 0898721c - Reapply "Try adding fiber support to the renderer."
• cd2cb325 - Cleanup

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added 1 commit

• 8433b85e - Fix iterations

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added 45 commits

• 8433b85e...78ec304c - 38 commits from branch project:11.x
• 87426373 - Try adding fiber support to the renderer.
• 9a86252d - Typo
• 864e081a - Revert "Typo"
• 9ecf2178 - Revert "Try adding fiber support to the renderer."
• 8d58640f - Reapply "Try adding fiber support to the renderer."
• 7ed38719 - Cleanup
• d41db014 - Fix iterations

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