Update OverdrawColorFilter to clarify how it interprets its input array.

1. Document that Make() takes RGBA_premul
2. Change impl so that CPU matches this behavior/definition
3. Offer a clearer factory that takes SkColor
   (perhaps migrate clients to this one in the future)

Bug: skia:9896
Change-Id: I0a81dda45834a2fbf2b28b711ba7e6b83f360aff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/269906
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed