* Rename clip mode to clip behavior

So we're consistent across flutter/flutter and flutter/engine

* Clang format

For flutter/flutter#18057

TEST=Compiled and ran flutter app on Fuchsia, verified it rendered and responded to input.

saveLayer is slow so we should avoid it as much as possible. If
there are artifacts without saveLayer, then we should report that
to Skia for the fix instead of slowing the performance with
saveLayer.

This PQ makes average rasterizer time drop from 25ms to 18ms in
flutter_gallery transitions perf test on a Nexus 5X.

This partially fixes flutter/flutter#13736 .
We probably still need some work in the opacity layer to squize
all the performance improvements.

* format

* license script adaptions

* updated licenses

* review comments

Fixes https://github.com/flutter/flutter/issues/9473

We push a bit up the tree during preroll to learn whether there system
composited layers below each layer. During update scene, we squash down
to paint tasks, which we execute after publishing the new scene.

The Skia team tells us that these go faster.

This patch removes almost all //base dependency of //flow. The only dependency
left is on tracing.

* Added instrumentation for timings.

* A few more traces for better granularity.

Instead, make Layer::Paint take a new PaintContext that has just exactly
the state that it needs, mirroring PrerollContext. Also, rename
PaintContext to CompositorContext because it holds the context for the
whole compositor.

We now compute the bounds for all container layers, including clips, and
use those bounds to tighten down our clips and save layers.

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

We need this to implement RenderShaderMask properly. See #1155.

The gr_context and canvas are stored at the frame level instead of the storing the same temporarily in the context

Make the paint context own the rasterizer and options. Also give it the ability to vend frames to render into.

dont mutate from frame to frame