Viewport fails to clip its AssetImage child

We were getting confused about our layer tree offsets, which caused us
to set an incorrect cull rect for the recording inside a viewport.

This patch does away with layer tree offsets almost entirely. We now use
them only at repaint boundaries, which is where we want the layer tree
to be mutable.

Fixes #1460

tell people to submit code after LGTM

Add scale awareness to images

Update to new editing.mojom

add --start-paused,--debug-port flags

add section on submitting code in CONTRIBUTING.md

add debugFillDescription to OverflowBox

Now the keyboard is responsible for maintaining the state of the text
field.

In addition to checking for the existence of the engine, ensure that it’s the correct version

refactor flutter logs

Semantics updates

Port the tests to use the new interface.

Update the Align docs

They were out-of-date.

Fixes #1391

fix a regex in the ios logs command

fixed sub-process stdout parsing; added todos

tweak the output for the analyze command

Adds the ability to move the caret by tapping

Now the text input control knows how to move the caret when you tap
inside the string. There's still some rough edges to polish up, but this
patch is the first step.

Fixes #108

Fix analyzer warnings related to SemanticsListener

These were introduced by the recent engine roll.

Input should paint selections

massage the help text