[skottie] Optimize opacity layerization

  - plumb a RenderContext RenderNode::render() argument to track pending
    opacity

  - defer opacity application until we can determine whether a layer is
    required (group with multiple children) or the opacity can be pushed
    onto the draw paint (for single/atomic draws)

Bug: skia:
Change-Id: I962ba08bad780395d5d738307bde986e9efa502b
Reviewed-on: https://skia-review.googlesource.com/146445
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
22 files changed