Today classes like EmbedderConfigBuilder and EmbedderTestBackingStoreProducer are full of #ifdefs for SHELL_ENABLE_METAL, SHELL_ENABLE_VULKAN, SHELL_ENABLE_GL etc.

Because this is all stock C++ code, we can't include Objective-C types in any headers transitively imported into any of them. This is problematic for Metal code since it precludes the usage of id<MTLDevice>, id<MTLCommandQueue> etc. in classes such as TestMetalContext, forcing us back to manual reference counting via __bridge_retained and __bridge_transfer.

Instead, we should break this code apart by backend, and implement the Metal code in Objective-C++.