add saturate helper for float2int64

Bug: skia:
Change-Id: I157f8c047145874c4585ae870018163908389b26
Reviewed-on: https://skia-review.googlesource.com/102321
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
4 files changed