Use the main thread for draw execution when single-threaded.

Bug 23073037

Change-Id: I47d5a7d80dc48dffae0014fbf619810290c30628
Reviewed-on: https://swiftshader-review.googlesource.com/3840
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>