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

MZ-259

* Update to new Dart FIDL bindings API.

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.

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

Fixes https://github.com/flutter/flutter/issues/7200

Apparently hittable is not politically correct.

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

There's no reason to present offscreen child views to Mozart.

Allow checkerboarding raster cache entries for debugging purposes (can be toggled from Dart). (#3200)

Example: $ mojo:launcher mojo:spinning_square

This patch removes the //flutter/sky/engine dependency from //flutter/lib/ui,
which lets us build the bulk of the dart:ui library without needing to build
//flutter/sky/engine.