Update skpbench to let the gpu thread run alongside the recording threads

Bug: skia:10176
Change-Id: Ibadd52b86abfd2802b9b3f31c86ab573554601db
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/289883
Commit-Queue: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed