fuchsia: create new flutter_runner render path

[arbreng]

Description

This new render path implements the ExternalViewEmbedder interface without using any legacy code, keeping all Scenic interaction contained to the embedder. I cribbed liberally from EmbedderExternalViewEmbedder when creating this.

Once stabilized, To-be-replaced by moving Fuchsia behind the formal C API in a series of follow-up CLs.

Related Issues

https://bugs.fuchsia.dev/p/fuchsia/issues/detail?id=46971

Tests

Ran workstation shell and internal tests w/ both render paths.

[gaaclarke]

I gave this a glance. I'm fine with it if everyone else is. I just had some style nits for the most part, maths seems fine. The thing that bothers me about it, if I may, is that it has placed a lot of conditional compilation all over the place. Which is especially bad because it is supporting legacy features and presumably will be removed at some point?

Alternatively a bit more care could be taken to reduce the number of #ifs. For example there could be 2 different SceneUpdateContext's:

static SceneUpdateContext* SceneUpdateContext::Create() {
#if defined(LEGACY_FUCHSIA_EMBEDDER)
  return SceneUpdateContextLegacy();
#else
  return SceneUpdateContextStandard();
#endif
}

This setup has the benefit of compiling the legacy code to make sure it doesn't atrophy. Keeps the standard implementation easier to read and makes the removal of the legacy code trivial. Also for other places where the #ifs are happening, if you wrap up the code in functions, the contract of the code is explicit which makes it easier to rip out or reason about. That's my take at least.

=)

[arbreng]

This setup has the benefit of compiling the legacy code to make sure it doesn't atrophy. Keeps the standard implementation easier to read and makes the removal of the legacy code trivial. Also for other places where the #ifs are happening, if you wrap up the code in functions, the contract of the code is explicit which makes it easier to rip out or reason about. That's my take at least.

=)

Newest version almost does this, although it's not quite as clean as what you suggested here. The complexity is contained to engine.cc though (which is the Fuchsia "embedder" and will be mutating rapidly).

[arbreng]

If this CL lands, it puts Fuchsia on parity with iOS/Android in terms of external view embedding!

[dreveman]

lgtm if gaaclarke is ok with latest version of this change

[dreveman]

This seems reasonable. We could clean it up more but as LEGACY_FUCHSIA_EMBEDDER is going away asap I think some extra code to keep it less risky for now is good.

[gaaclarke]

Wow, I almost don't even recognize it. FuchsiaExternalViewEmbedder worked out great. I think the #ifs are in a better position now. LGTM