Rendering on the main thread can cause segmentation faults on Windows
due to uncommitted stack memory, since Subzero does not currently call
__chkstk. In practice this would have mainly been an issue on VMs with
a single virtual CPU.

Bug swiftshader:25

Change-Id: Ic3be7e5a41ef09b7e056d3c3df2983c225101fe4
Reviewed-on: https://swiftshader-review.googlesource.com/8934Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>