[sksg] Stop caching full SkPaints in Paint nodes

Create on the fly, when needed.

TBR=
Bug: skia:8340
Change-Id: I4c319821a6d6e84e0ec37e8858c4a89ba1668626
Reviewed-on: https://skia-review.googlesource.com/153141
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed