Don't freeze devicePixelRatio at startup

We were making local copies of the listener maps, but we were actually
iterating the underlying sets. Now we make local copies of the sets.

Fixes #803

I removed these functions by mistake in my previous patch.

When we start, we might not have initialized the devicePixelRatio value. The
value will be initialized by the time we paint, so now we update the value
every time we paint.

Use sky.SceneBuilder to upload Layer tree to C++

We still use SkPicture as our rasterization backend, but now we're uploading
our layer tree to C++. A future patch will push the layer tree deeper into the
system and we'll eventually switch backends.

SnackBar's crash if you fail to provide a showing bool.

I tried to edit it in the framework, but this seemed easier for now.

The snackbar still shows behind the keyboard unfortunately.
https://github.com/domokit/sky_engine/issues/810

@collinjackson

@collinjackson

I'm not sure this is the correct approach, but it
at least mitigates https://github.com/domokit/sky_engine/issues/804
for now.

I also made it not crash if you haven't set a goalWeight.

@collinjackson

Added Scrollable named parameter initializeScrollOffset. If unspecified, then the initial value of scrollOffset is 0.0 as before.

Additions to demo game

Don't divide by zero

If there aren't any items, there's no point in computing which items to show.

Add `Row` and `Column` widgets so you don't have to use Flex.

Remove floating-point errors from ScrollableWidgetList

We now compute which items to show using integers instead of floating point,
avoiding rounding errors.

Minor code cleanup

I updated everything in widgets/, but didn't update any examples.

New demo game

Minor cleanup in sky_tool

...rather than the stack trace at the time the RenderObjectWrapper updates the RenderObject.

Also, hide some of the debug-only fields behind "debug" prefixes and assert()s.

Better exception handling for rendering library.

Fix sky_tool listen on linux.

- Catch exceptions closer to the source.
- Factor out exception printing code.
- Have widget library hand the rendering library some context when syncing RenderObjectWrappers to aid with debugging.
- Fix a bug in flex.dart whereby _overflow was compared when null.

Mimic should track Mimicable more completely

We now have Mimic tracking Mimicable through tree structure changes and while
moving around the screen.

Fixes #751 and #756

Fix typography to use opacity instead of color value