Cocoa: Add basic support for Vulkan via MoltenVK

This adds basic support for MoltenVK, a Vulkan implementation on top of
Metal, on macOS 10.11 and later.  It looks for MoltenVK in the process
via RTLD_DEFAULT symbol lookup if _GLFW_VULKAN_STATIC is disabled.

glfwCreateWindowSurface now creates and sets a CAMetalLayer for the
window content view, which is required for MoltenVK to function.

You must help CMake find MoltenVK for the Vulkan test to be built.

Fixes #870.

tests/CMakeLists.txt

@@ -50,7 +50,7 @@ if (VULKAN_FOUND)
     add_executable(vulkan WIN32 vulkan.c ${ICON})
     target_include_directories(vulkan PRIVATE "${VULKAN_INCLUDE_DIR}")
     if (NOT GLFW_VULKAN_STATIC)
-        target_link_libraries(vulkan "${VULKAN_LIBRARY}")
+        target_link_libraries(vulkan "${VULKAN_LIBRARY}" ${GLFW_VULKAN_DEPS})
     endif()
     list(APPEND WINDOWS_BINARIES vulkan)
 endif()