This patch sketches out the basic widgets for creating Mozart child
views. We're still missing some of the machinery, so we can't quite
create child views yet, but once we supply the ViewHost and teach the
compositor how to actually display the child scenes, we'll have
something that works.

We were getting confused about our layer tree offsets, which caused us
to set an incorrect cull rect for the recording inside a viewport.

This patch does away with layer tree offsets almost entirely. We now use
them only at repaint boundaries, which is where we want the layer tree
to be mutable.

Fixes #1460

Also, update callers of deprecated methods.

We no longer need to pass bounds to SceneBuilder. Instead, the
compositor computes these bounds itself from the RTrees in the
SkPictures.

This patch adds ShaderLayer and makes ShaderMask use it.

Fixes #1155

We don't know how to accuately compute paint bounds in the render tree.
Instead, we can rely on the compositor to compute the paint bounds for
us if we use OpacityLayer to do our opacity blends.

Fixes the shadow when closing the menu in the stocks app.

We weren't correctly offsetting the clips when uploading composited scenes. Now
we apply the proper offsets.

To help debugging issues with the layer tree.

Now we use a ForcedLayer in the navigator overlay to cache the recording for
each entry in the overlay. This mechanism just caches the display list, not the
underlying pixels.

Also, remove the "dispose" notification in the Layer tree because it was
disposing layer too eagerly. We don't actually need this notification for
anything other than eagerly freeing some C++ memory.

This patch calls dispose() explicitly on a number of heavy C++ objects so that
they can be eagerly finalized instead of waiting for GC.

This widget is slow and not commonly used. The original reason we added it was
to colorize images, but now that feature is built into the image and background
image widgets.

Fixes #1646

The goal is to follow the guidelines in
https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md#packages

Fixes #1638

Example:
  import 'package:sky/widgets.dart';
  void main() => runApp(new Center(child:
      new StatisticsOverlay.allEnabled()));

* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

Code outside of package:sky should import this code using

package:sky/rendering.dart
package:sky/widgets.dart

Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.

We still use SkPicture as our rasterization backend, but now we're uploading
our layer tree to C++. A future patch will push the layer tree deeper into the
system and we'll eventually switch backends.

We now use the repaint system to do all the painting. During initialization, we
set up a root layer that applies the device pixel ratio.

Fixes #706

The compositor backends we're planning to use can't handle a general-purpose
paint layer and instead need lower-level operations.

Fixes #707

- Introduce some new Layer classes.
- Introduce paintChildWith* methods.
- Convert paint() methods to use paintChildWith* where appropriate.
- Fix paintBounds logic in Layer world.
- Introduce Layer.replaceWith(), so that it's clearer what's going on.
- Make RenderObjects have a ContainerLayer, not a PictureLayer.
- Introduce a PaintingContext.replacingLayer() constructor to highlight
  where we are creating a layer just to replace an older one.
- Rename some layer-related methods and fields for clarity:
   requiresCompositing -> hasLayer
   hasCompositedDescendant -> needsCompositing
   updateCompositing -> updateCompositingBits
   _needsCompositingUpdate -> _needsCompositingBitsUpdate
   markNeedsCompositingUpdate -> markNeedsCompositingBitsUpdate
- After updating compositing bits, if we find that the bit changed, we
  now call markNeedsPaint().
- Reorder markNeedsPaint() logic for clarity.
- Make flushPaint() start at the deepest node.
- Make _compositeChild() avoid repainting children with hasLayer that
  aren't dirty, instead it just reuses their existing layer.
- Made RenderView reuse the RenderObject layer instead of having its own.
- Made RenderView have hasLayer set to true.
- Add various asserts and comments.

- Add Canvas.getSaveCount()
- Make RenderClipRect call context.paintChildWithClip instead of doing the clipping itself
- Make ClipLayer take a Rect instead of a Size
- Make PaintingContext.canvas read-only
- Add PaintingContext.paintChildWithClip()
- Minor rearrangings of code and style tweaks

This patch makes it easier to debug layerization issues.

Separating these notions makes them easier to work with because offset is
relative to the parent layer whereas size is intrinsic to the layer itself.
This patch fixes the underpainting bugs when compositing the stocks example.

Now we have the ability to draw multiple PictureLayers. We still squash all the
pictures together into a single SkPicture for simplicity. In the future, we'll
submit them to C++ separately and composite them on the GPU thread.

Currently we have a single PictureLayer that everyone draws into. A future
patch will teach the system to use multiple PictureLayers.