[SVGDom] Opacity optimization

Apply opacity as fill/stroke paint alpha instead of saveLayer, when
possible.

R=robertphillips@google.com,stephana@google.com,reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2353503005

Committed: https://skia.googlesource.com/skia/+/3dbb7b9f196d793fbd16243157ee67638891f5dc
Review-Url: https://codereview.chromium.org/2353503005
6 files changed