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.