dfb631ffed
Extracts backend-specific code in EmbedderConfigBuilder to separate translation units. In particular, this allows for less conditional header includes, and more specifically, for code relating to the Metal backend to include headers that include Objective-C types -- today we cast these all to void* to avoid declaring them in headers, which requires special handling for ARC. An alternative approach would have been to extract backend-specific subclasses, but there are test suites such as EmbedderTestMultiBackend that are executed against multiple backends, which currently make that approach impractical, though that should likely be the long-term goal. Issue: https://github.com/flutter/flutter/issues/137801 [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style