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.

docs/compat.dox

@@ -185,10 +185,11 @@ a non-default value will cause @ref glfwCreateWindow to fail and the
 
 @section compat_vulkan Vulkan loader and API
 
-GLFW uses the standard system-wide Vulkan loader to access the Vulkan API.
-This should be installed by graphics drivers and Vulkan SDKs.  If this is not
-available, @ref glfwVulkanSupported will return `GLFW_FALSE` and all other
-Vulkan-related functions will fail with an @ref GLFW_API_UNAVAILABLE error.
+By default, GLFW uses the standard system-wide Vulkan loader to access the
+Vulkan API on all platforms except macOS.  This is installed by both graphics
+drivers and Vulkan SDKs.  If the loader is not found, @ref glfwVulkanSupported
+will return `GLFW_FALSE` and all other Vulkan-related functions will fail with
+an @ref GLFW_API_UNAVAILABLE error.
 
 
 @section compat_wsi Vulkan WSI extensions
@@ -201,6 +202,11 @@ surfaces on Microsoft Windows.  If any of these extensions are not available,
 @ref glfwGetRequiredInstanceExtensions will return an empty list and window
 surface creation will fail.
 
+GLFW uses the `VK_KHR_surface` and `VK_MVK_macos_surface` extensions to create
+surfaces on macOS.  If any of these extensions are not available, @ref
+glfwGetRequiredInstanceExtensions will return an empty list and window surface
+creation will fail.
+
 GLFW uses the `VK_KHR_surface` and either the `VK_KHR_xlib_surface` or
 `VK_KHR_xcb_surface` extensions to create surfaces on X11.  If `VK_KHR_surface`
 or both `VK_KHR_xlib_surface` and `VK_KHR_xcb_surface` are not available, @ref
@@ -217,8 +223,4 @@ surfaces on Mir.  If any of these extensions are not available, @ref
 glfwGetRequiredInstanceExtensions will return an empty list and window surface
 creation will fail.
 
-GLFW does not support any extensions for window surface creation on macOS,
-meaning@ref glfwGetRequiredInstanceExtensions will return an empty list and
-window surface creation will fail.
-
 */

docs/news.dox

@@ -16,6 +16,12 @@ GLFW now supports querying the platform dependent scancode of any key with
 @ref glfwGetKeyScancode.
 
 
+@subsection news_33_moltenvk Support for Vulkan on macOS via MoltenVK
+
+GLFW now supports the `VK_MVK_macos_surface` window surface creation extension
+provided by MoltenVK.
+
+
 @section news_32 New features in 3.2
 
 

docs/vulkan.dox

@@ -11,8 +11,10 @@ with Vulkan concepts like loaders, devices, queues and surfaces and leaves it to
 the Vulkan documentation to explain the details of Vulkan functions.
 
 To develop for Vulkan you should install an SDK for your platform, for example
-the [LunarG Vulkan SDK](https://vulkan.lunarg.com/).  Apart from the headers and
-libraries, it also provides the validation layers necessary for development.
+the [LunarG Vulkan SDK](https://vulkan.lunarg.com/) for Windows and Linux or
+[MoltenVK](https://moltengl.com/moltenvk/) for macOS.  Apart from headers and
+link libraries, they should also provide the validation layers necessary for
+development.
 
 The GLFW library does not need the Vulkan SDK to enable support for Vulkan.
 However, any Vulkan-specific test and example programs are built only if the
@@ -28,6 +30,26 @@ are also guides for the other areas of the GLFW API.
  - @ref input_guide
 
 
+@section vulkan_loader Linking against the Vulkan loader
+
+By default, GLFW will look for the Vulkan loader on demand at runtime via its
+standard name (`vulkan-1.dll` on Windows, `libvulkan.so.1` on Linux and other
+Unix-like systems).  This means that GLFW does not need to be linked against the
+loader.  However, it also means that if you are using the static library form of
+the Vulkan loader GLFW will either fail to find it or (worse) use the wrong one.
+
+The [GLFW_VULKAN_STATIC](@ref compile_options_shared) CMake option makes GLFW
+link directly against the static form.  Not linking against the Vulkan loader
+will then be a compile-time error.
+
+@macos MoltenVK only provides the static library form of the Vulkan loader, but
+GLFW is able to find it without
+[GLFW_VULKAN_STATIC](@ref compile_options_shared) as long as it is linked into
+any of the binaries already loaded into the process.  As it is a static library,
+you must also link against its dependencies: the `Cocoa`, `Metal` and
+`QuartzCore` frameworks and the `libc++` library.
+
+
 @section vulkan_include Including the Vulkan and GLFW header files
 
 To include the Vulkan header, define [GLFW_INCLUDE_VULKAN](@ref build_macros)