support init engine asynchronous

[lybeen]

Description

In Engine initialization, we have to wait for all other threads ready, it has an opportunity to improve it, so I make this asynchronous.

Comparison

Before

After

Conclusion:

It's clearly show that it makes ui thread asynchronous , platform thread does't have to wait for it in engine initialization.
It may make about 200ms time for platform thread.

Related Issues

19641

This pr seems a little hackable, my solution seems a bit more sensible.

Tests

I added the following tests:

Go through all test cases

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 [C++, Objective-C, Java style guides] for the engine.
• I read the [tree hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read [handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.

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[lybeen]

@googlebot I fixed it.

[chinmaygarde]

cc @jason-simmons @liyuqian

[liyuqian]

This approach looks similar to #18047. That PR is very close to pass all the tests but there's some mysterious "Windows Host Engine" failure. @lybeen : can you please check if that PR is helpful in your case, and whether you're able to revise your PR to pass all tests?

[gaaclarke]

I've worked with this issue in the past with another contributor. There are too many places where we assume we have an initialized Shell. I don't want to create the opportunity to create bugs by allowing shell to be in yet another state.

I think this is a good goal but the change should use std::future to represent a shell that hasn't been initialized yet so that we can safely migrate the code to using it. The previous contributor did most of the work but ran into memory problems on windows which they didn't address so it stalled out.

Edit: the other PR: #18047

[chinmaygarde]

Closing this as we want to go with another approach.