[SVGDom] Avoid some unneeded canvas save()s

The canvas only needs to be saved once, per local SkSVGRenderContext.

Add a helper (saveOnce) to implement this optimization.

Change-Id: I0c21fa78ad9fd5d3d11de0a29f8441620488d676
Reviewed-on: https://skia-review.googlesource.com/58340
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Robert Phillips <robertphillips@google.com>
5 files changed