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.

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

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.

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 relying upon the rects passed in from Dart, the compositor
should compute the paint bounds of layers from the cull rects of the
underlying SkPictures. This approach is better because it will handle
effects like shadows that paint outside the incorrect paint bounds we
use today (as well as shrinking around empty space).

Prerolling the rasterization tasks reduces the number of render target
switches because we don't interrupt the main render target.

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