compute center slightly slower to avoid overflow

Bug: oss-fuzz:8509
Change-Id: I13b1a77e1549070827a7cc534b062ec85aad255e
Reviewed-on: https://skia-review.googlesource.com/129930
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed