Add colorFilterLayer/Widget

[dnfield]

Description

Adds a ColorFilter layer and widget. customer: octopod would like to use this to preview color blindness filters on widget trees.

Depends on flutter/engine#9641 - tests will fail until that rolls in.

Related Issues

Fixes #5692

Tests

I added the following tests:

Test that ColorFilterLayer does the right thing when it changes.

Golden tests for ColorFiltered widget.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
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Breaking Change

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• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change (though the upstream engine change is).

[dnfield]

/cc @emshack - this should enable doing the color filter previewing of widget trees. It's waiting on the upstream engine change, which should hopefully land soon.

[collinjackson]

lgtm modulo minor documentation nit

[dnfield]

Oh man I could have sworn the build was green when I merged this. Whups.