This patch moves the remainder of painting and compositing into their
new home (and updates their style to match Google C++ Style).

Now that tonic doesn't depend on anything in //sky/engine anymore, we
can move the code to a location where its dependencies are clearer.

This patch paves the way to untangling a large amount of code from the engine.

Now that the compositor doesn't have any tricky deps, we can make it a
top-level project, which saves a bunch of typing.

Rather than using IDL code generation, we now use some template magic to
generate this code. No all the code is template-generated yet, but this
patch is a start in that direction.

Some host objects hold on to a lot of state and take time to tear down. This
patch adds the ability to dispose them eagerly to release resources faster.

dont mutate from frame to frame

Instead of squashing the layers down into a single SkPicture, we now pass the
tree to the GPU thread, which draws everything separately.

We now build a layer tree before squashing the draw command down to an
SkPicture.

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.

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.

R=abarth@chromium.org, abarth@google.com, ianh@google.com

Review URL: https://codereview.chromium.org/1221583003.

Decouple Canvas from DisplayList and map Picture and PictureRecorder more directly to their Skia counterparts.

Also changes the framework dart code to use the
refactored APIs and fixes the various examples and
tests.

R=abarth@chromium.org, ianh@chromium.org

Review URL: https://codereview.chromium.org/1190123003.

This CL adds a global view object that can receive a Picture and be signaled to
draw. When using SkyView, this Picture shows up on screen.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1134913003

This adds a new abstract 'Canvas' which is similar to SkCanvas.
PaintContext implements Canvas while still having its
own commit() method to cause the paint actions to apply to
the Element for the next frame.
This adds a new PictureRecorder which also implements Canvas
and has an endRecording() method which returns a Picture
(another new interface) which can be held from Dart.

There is also now a rootPicture setter on Document which takes
a Picture and will then make the Document draw that Picture
until changed.

This piggybacks on the existing custom painting system
which adds the painting at background-and-borders paint
time so technically if you both set rootPicture as well as
construct a DOM you will draw the DOM on top of your picture. :)

R=mpcomplete@chromium.org

Review URL: https://codereview.chromium.org/1122423009