[SPv2] Respect transform-style in the Blink and cc property trees.

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

 # Platform paint code
 
 This directory contains the implementation of display lists and display
 list-based painting, except for code which requires knowledge of `core/`
 concepts, such as DOM elements and layout objects.
 
 This code is owned by the [paint team][paint-team-site].
 
 Slimming Paint v2 is currently being implemented. Unlike Slimming Paint v1, SPv2
 represents its paint artifact not as a flat display list, but as a list of
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 which defines the coordinate space in which the content should be painted.
 
 Each transform node has:
 
 * a 4x4 [`TransformationMatrix`](../../transforms/TransformationMatrix.h)
 * a 3-dimensional transform origin, which defines the origin relative to which
   the transformation matrix should be applied (e.g. a rotation applied with some
   transform origin will rotate the plane about that point)
 * a pointer to the parent node, which defines the coordinate space relative to
   which the above should be interpreted
+* a boolean indicating whether the transform should be projected into the plane
+  of its parent (i.e., whether the total transform inherited from its parent
+  should be flattened before this node's transform is applied and propagated to
+  children)
+* an integer rendering context ID; content whose transform nodes share a
+  rendering context ID should sort together
 
 The parent node pointers link the transform nodes in a hierarchy (the *transform
 tree*), which defines how the transform for any painted content can be
 determined.
 
 ***promo
 The painting system may create transform nodes which don't affect the position
 of points in the xy-plane, but which have an apparent effect only when
 multiplied with other transformation matrices. In particular, a transform node
 may be created to establish a perspective matrix for descendant transforms in
 order to create the illusion of depth.
 ***
 
-*** aside
-TODO(jbroman): Explain flattening, etc., once it exists in the paint properties.
-***
+Note that, even though CSS does not permit it in the DOM, the transform tree can
+have nodes whose children do not flatten their inherited transform and
+participate in no 3D rendering context. For example, not flattening is necessary
+to preserve the 3D character of the perspective transform, but this does not
+imply any 3D sorting.
 
 ### Clips
 
 Each paint chunk is associated with a [clip node](ClipPaintPropertyNode.h),
 which defines the raster region that will be applied on the canvas when
 the chunk is rastered.
 
 Each clip node has:
 
 * A float rect with (optionally) rounded corner radius.
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 visual and transformed rects of display items in the coordinate space of ancestor
 [`PropertyTreeState`](PropertyTreeState.h)s.
 
 The transformed rect of a display item in an ancestor `PropertyTreeState` is that
 rect, multiplied by the transforms between the display item's `PropertyTreeState`
 and the ancestors, then flattened into 2D.
 
 The visual rect of a display item in an ancestor `PropertyTreeState` is the intersection
 of all of the intermediate clips (transformed in to the ancestor state), with
 the display item's transformed rect.