pass coverage as [0,1] float

Change-Id: I4bc99b59c8c845cd030c23a2204d07d38e0e1ccd
Reviewed-on: https://skia-review.googlesource.com/c/eskia/+/211578
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed