Only support main thread rendering in debug builds.
Only support main thread rendering in debug builds.

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/8934
Tested-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Reviewed-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>

bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader