tweak screen math

Seems to help get past our r,g,b <= a asserts better.
I guess it keeps more precision from the small s*d term?

    Old:  (s+d) - (s*d)
    New:  s + (d - (s*d))

Change-Id: Ib9b9472e2c08aef688bbfdbccaf0bd45b7bf3ecd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268592
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
1 file changed