limit n7/np to main thread

with a little refactoring around --threads

Change-Id: I8dfb3f3c7344969842e6c566fcdfb3a35d934951
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201877
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed