This fixes two small races we had while simultaneously reading from and
writing to the vertex timestamp, and simultaneously writing to the
vertex isStateOK (dirty) flag.
They were actually "safe" races, in that it doesn't matter if the
read/write race got the old or new value, or that the double write
happened. The time sequencing was correct (I believe) in both cases, but
this triggers the race detector now that we have tests for it.
c2f508e261