layout: Disable parallel display list building by default.
I've never see it result in a speedup. Actually, I don't think I've seen
it result in anything better than a 50% slowdown. The arithmetic
intensity is just too low, at least with the current algorithm.
Parallel DL building can still be enabled with a debug flag if the
algorithm is improved.
r? @metajack
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b876a54dce
