Use SkTPin instead of std::clamp

std::clamp is c++17 so using it causes some builds to fail.

Change-Id: I3e90ffc081e1f18ebab1c4f403826e94e58bf07d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299878
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>