Implement merging non-composited paint property nodes in the PACompositor.

third_party/WebKit/Source/platform/graphics/compositing/README.md

+# `Source/platform/graphics/compositing`
+
+This directory contains the implementation of the "Blink compositing algorithm".
+
+This code is owned by the [paint team][paint-team-site].
+[paint-team-site]: https://www.chromium.org/developers/paint-team
+
+This document explains the SPv2 world as it develops, not the SPv1 world it
+replaces.
+
+## Blink compositing algorithm
+
+Design document: goo.gl/6xP8Oe
+
+Inputs: `PaintArtifact`
+Outputs: List of `cc::Layer` objects and `cc::PropertyTree`'s.
+
+The algorithm walks through the list of `PaintChunks` in the `PaintArtifact`,
+allocating new `c::Layers` if the `PaintChunk` cannot merge into an existing
+`cc::Layer`. The reasons why it would not be able to do so are:
+
+1. The `PaintChunk` requires a foreign layer (see below)
+
+2. The `PaintChunk` cannot merge with any existing layer, due incompatible
+direct compositing reasons on its `PropertyTreeState`.
+
+3. The `PaintChunk` overlaps with an earlier `cc::Layer` that it can't merge with
+due to reason 2, and there is no later-drawn `cc::Layer` for which reasons 1 and
+2 do not apply.
+
+In the worst case, this algorithm has an O(n^2) running time, where n is the
+number of `PaintChunks`.
+
+All property tree nodes referred to by any `PaintChunk` are currently copied
+into their equivalent `cc::PropertyTree` node, regardless of whether they are
+required by the above.
+
+### Flattening property tree nodes
+
+When `PaintChunks` can merge into an existing `cc::Layer`, they may have
+different `PropertyTreeState`s than the `PropertyTreeState` of the `cc::Layer`.
+If so, we need to *flatten* down the nodes that are different between the
+`PropertyTreeState` of the `PaintChunk` and the `cc::Layer`. This is done
+by iterating over the "innermostNode" of the `PropertyTreeState`, finding
+the sequence of transform, clip and effect nodes that need to be flattened,
+and turning them into a sequence of paired display items representing the
+transforms, clips and effects. The algorithm for this is spelled out in
+the designdoc at http://goo.gl/6xP8Oe.
+
+### Foreign layers
+
+Some `PaintChunk` content requires a foreign layer, meaning a layer that is
+managed outside of the scope of this code. Examples are composited video, a
+and 2D/3D (WebGL) canvas.
+
+### Raster invalidations
+
+Any raster invalidates on a `PaintChunk` are also mapped to the space of the
+backing `cc::Layer`.