add to_fp16/from_fp16

Nothing too tricky here.

Change-Id: I48e51c301e53efc63fc92c378fe45a0e5a2df7e6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/340520
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed