Add VK_EXT_external_memory_metal #987

Job	Run time
Build the core-only spec, to try and catch ifdef errors in extension markup	54s
Verify repository license compliance	40s
Run consistency checks on the spec and XML sources	54s
Run Vulkan CTS framework tests to validate against XML changes	1m 40s
Build the vulkan specification and generate any associated files (such as vulkan.h)	6m 17s
Build a test spec and check against expectation, to make sure the spec toolchain works as expected	1m 21s
Generate Rust bindings (Ash crate)	57s
Compile a simple test program that uses vulkan.h	9s
Generate the vulkan C++ header (vulkan.hpp)	1m 2s
Build-test Rust bindings (Ash crate)	43s
Compile a simple test program that uses vulkan.hpp	22s
	14m 59s

Provide feedback