Add VK_EXT_external_memory_metal #990

Job	Run time
Run consistency checks on the spec and XML sources	54s
Verify repository license compliance	37s
Build the core-only spec, to try and catch ifdef errors in extension markup	48s
Run Vulkan CTS framework tests to validate against XML changes	1m 42s
Build the vulkan specification and generate any associated files (such as vulkan.h)	6m 24s
Build a test spec and check against expectation, to make sure the spec toolchain works as expected	1m 13s
Generate Rust bindings (Ash crate)	33s
Compile a simple test program that uses vulkan.h	8s
Generate the vulkan C++ header (vulkan.hpp)	1m 19s
Build-test Rust bindings (Ash crate)	40s
Compile a simple test program that uses vulkan.hpp	25s
	14m 43s

Provide feedback