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.

Rather than receiving an ApplicationEnvironment, we now receive a namespace.
Once we have FIDL2, we can plumb the namespace all the way through to Dart.

We still don't get IME text, but at least we don't crash.

This patch wires Mozart's text input system to flutter/textinput. This patch
doesn't fully work yet because the Mozart side isn't fully implemented.

Instead of updating to match new HID touchscreen hacks exposed by
magenta, just remove the direct input layer entirely.  Mozart all the
time from now on.

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

* New mozart input events.

* Adding TODO for keyrepeat

This interface has been replaced with HandleWaiter, which is the new hotness.

These are implemented on macOS and Fuchsia.

Now we just pass the data directly.

After this patch, platform messages now take the same path through the system
that semantics data does (on Android). Support on iOS will be in another patch.

This patch prepares us to switch sending semantics information over mojom to
using dart:ui directly. Currently the recorded information is dropped on the
floor, but that will improve in future patches.

Example: $ mojo:launcher mojo:spinning_square

After this patch, Flutter apps on Fuchsia can access their asset
bundles. Unlike other platforms, the asset bundles are stored in memory
rather than in the file system.

After this patch, we're able to run spinning_square on Fuchsia. I've
left some TODO comments about some shortcuts I've taken that we'll need
to improve in the future.

This patch should contain all the code we need to run Flutter on
Fuchsia's software framebuffer (without text). I haven't actually tried
running the code, so I'm sure it doesn't work yet.