Add Support for Z-Index on CanvasKit PlatformView Layer, Don't Require Additional WebGL Contexts for Layers Drawn Behind the Canvas

[jezell]

Implements flutter/flutter#123277

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

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[Hixie]

@jezell Thanks for your contribution. Unfortunately without the CLA being signed we cannot consider your code. I'm going to close this PR as per the bot's comment above, but please don't hesitate to resubmit a PR if you are ever in a position to sign the CLA. Thanks!

[jezell]

@Hixie I did sign the CLA...

[Hixie]

Ah, I see. It looks like your commits had multiple e-mail addresses and that confused the bot, which marked it as unsigned, which caused everyone on the team to ignore the PR. Can you resend the patch with all your commits coming from the e-mail you have signed with, as a new PR? Then we can get this routed for actual code review. Sorry for all the hoop jumping, the lawyers insist on this process.