c74f169f93
Since Metal code frequently uses Objective-C types like id<MTLTexture> etc. this migrates Metal translation units to Obj-C++. In particular, this allows transitively including headers that include such types. This in turn helps with refactoring `EmbedderTest`, `TestMetalContext`, `TestMetalSurface` to avoid specifying `void*` types in headers and manually refcounting via ARC bridge casts. Reformats the source, since Objective-C files are linted to 100 columns rather than the 80 column limit we impose on C++ files. This change introduces no semantic changes. Issue: https://github.com/flutter/flutter/issues/137801