add savelayer flag to promote the later to half-float

Bug: skia:9256
Change-Id: Ifd7846349f83854e860306bd475cfba0adeddcb5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227066
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
3 files changed