Rename RenderCanvas to PaintingCanvas to avoid confusion with other classes that inherit from Rende…

sky/sdk/lib/rendering/README.md

 Sky Rendering
 =============
 
 The Sky render tree is a low-level layout and painting system based on a
 retained tree of objects that inherit from [`RenderObject`](object.dart). Most
 developers using Sky will not need to interact directly with the rendering tree.
 Instead, most developers should use [Sky widgets](../widgets/README.md), which
 are built using the render tree.
 
 Overview
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  * Implementations of `performLayout` are expected to call `layout` on their
    children. When calling `layout`, a `RenderObject` must use the
    `parentUsesSize` parameter to declare whether its `performLayout` function
    depends on information read from the child. If the parent doesn't declare
    that it uses the child's size, the edge from the parent to the child becomes
    a _relayout boundary_, which means the child (and its subtree) might undergo
    layout without the parent undergoing layout.
 
  * Subclasses of `RenderObject` must implement a `paint` function that draws a
-   visual representation of the object onto a `RenderCanvas`. If
+   visual representation of the object onto a `PaintingCanvas`. If
    the `RenderObject` has children, the `RenderObject` is responsible for
-   painting its children using the `paintChild` function on the `RenderCanvas`.
+   painting its children using the `paintChild` function on the
+   `PaintingCanvas`.
 
  * Subclasses of `RenderObject` must call `adoptChild` whenever they add a
    child. Similarly, they must call `dropChild` whenever they remove a child.
 
  * Most subclasses of `RenderObject` will implement a `hitTest` function that
    lets clients query the render tree for objects that intersect with a given
    user input location. `RenderObject` itself does not impose a particular
    type signature on `hitTest`, but most implementations will take an argument
    of type `HitTestResult` (or, more likely, a model-specific subclass of
    `HitTestResult`) as well as an object that describes the location at which
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 * Avoid using save/restore on canvases.
 
 
 Dependencies
 ------------
 
  * [`package:sky/base`](../base)
  * [`package:sky/mojo`](../mojo)
  * [`package:sky/animation`](../mojo)