Revert "Merge pull request #2609 from brson/parallel-render"

This reverts commit 850bd2891de589b95e32dc8f0b59d4043ed1e0a3, reversing
changes made to 5b0feac32a3ef13a76670eb2f5f21e7b598fc890.
Merge pull request #2685 from larsbergstrom/revert_2609

Revert "Merge pull request #2609 from brson/parallel-render"