[skottie] Tweak skottie_tool --threads meaning
[skottie] Tweak skottie_tool --threads meaning

There's an implicit +1 to the SkTaskGroup worker count, as the
main/scheduling thread also participates in the queue.

This is quite confusing.

Adjust the --threads parameter such that 1 means single threaded,
2 means rendering on two threads, etc.

Change-Id: Ib55c4b0a29cb342c7d18a15384e1c511614ee7af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/243740
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>

aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc