Fixes https://github.com/domokit/sky_engine/issues/540

@abarth

Hello world pubspec

Address book pubspec

Simplify SkPicture

This patch simplifies the SkPicture we generate for Skia. Instead of drawing
everything into a nested SkPicture, we now draw everything into the top-level
picture, which requires us to apply the device scale factor in Dart.

Move main.dart to lib/main.dart for the game

This required me wrapping the new Activity APIs
as well as adding the missing pubspec.yaml

@abarth

Adds new Layer class to sprites

Fix tab indicator animation so it doesn't snap to the previous tab.

If you tap multiple tabs in a row, the tab animation used to snap to the
last selected tab when starting a new animation. Fix that.

Also use the BuilderTransition so we don't have to rebuild the tab bar
every frame.

Minedigger pubspec

Optimize circular clips slightly

This patch avoids creating weak references on every paint call by caching the
Path object between paints.

Just infer it from the sign of the velocity. That's what odeon does.

Move examples to the root

Fix asserts related to image drawing.

There were two problems:

1) When starting the mimic, we put up a bad frame because although we set the
   `begin` value of the animation, we were building using the current `value`,
   which hadn't been updated.

2) When stoping the mimic, we'd dirty a component during didUnmount, which
   wouldn't get cleaned until the next frame. Now we're sure to clean all the
   components before leaving flushBuild.

Also, make RenderStack support negative positions with clipping so that we can
expand objects that are partially offscreen.

First example pubspec

Specify local build paths when using sky_tool.

directory to support using sky_tool with local
builds.

Add a --checked argument to sky_tool start