Ensure that a Mozart EntityNode (that corresponds
to an ExportNode) is always released on the
Rasterizer thread.

MZ-259

Compute the necessary texture resolution using more accurate scaling
information provided by Mozart scene node metrics events instead of the
device pixel ratio provided by the Mozart view properties (which we
might remove in the future).

This allows us to allocate smaller textures when a Flutter view is
being scaled down.

Instead of flushing each canvas, flush once on the context from which each of the canvas is derived.
This gives us a significant performance increase on the fuchsia vulkan backend

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

This version does not recycle buffers. It is meant only as an initial solution,
and the proper buffer recycling logic has been broken out into a separate change

Also, change the hit testing geometry so that we get our hits at the root.

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.