add loadF() and storeF() helpers to Builder

Change-Id: I5fe1ca090868cfb9fa930753232e9cbb42737d9a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278473
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed