Fix for saveLayer() with filters vs. the BBox Hierarchy.

Fix for saveLayer() with filters vs. the BBox Hierarchy.

When building acceleration structures for SkPicture, we must transform
the primitive's bounds not only by its own SkPaint, but by the paints of
any saveLayer()s currently active above it.

We do this by pushing the SkPaint onto a stack on
saveLayer(), and popping them on restore(). We also push
a NULL paint in save(), so that the pushes and pops are
balanced.

BUG=skia:2734
R=mtklein@google.com, reed@google.com, robertphillips@google.com

Committed: https://skia.googlesource.com/skia/+/6ca0b6a46cbe9bef3e2b9b9db813ec864efd62de

Author: senorblanco@chromium.org

Review URL: https://codereview.chromium.org/380373003