387ca2ef86
When we visit a PlatformViewLayer during the paint traversal it replaces the PaintContext's canvas with a new one that is painted ontop of the embedded view. We need to make sure that operations applied by parent layers are also applied to the new canvas. To achieve this we collect all the canvases in a SkNWayCanvas and use this canvas by non leaf nodes. Leaf nodes still paint only to the "current" canvas. This PR moves the overlay canvas creation from the paint phase to the preroll phase, collects them into a SkNWayCanvas and set it in PaintContext. To keep this PR focused, I only used the internal_nodes_canvas in the tranform_layer. Will followup with a PR that changes all internal layers to use the internal_nodes_canvas.
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