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.
Added GLFW_INCLUDE_VULKAN. Added glfwVulkanSupported,
glfwGetRequiredInstanceExtensions, glfwGetInstanceProcAddress,
glfwGetPhysicalDevicePresentationSupport and glfwCreateWindowSurface.
Added port of LunarG SDK tri example.
