Add glfwInitVulkanLoader

This removes the GLFW_VULKAN_STATIC CMake option and the
_GLFW_VULKAN_STATIC configuration macro and replaces them with the
glfwInitVulkanLoader function, allowing a single library binary to
provide both behaviors.

This is based on the design from PR #1374 by @pmuetschard.

Closes #1374
Closes #1890

include/GLFW/glfw3.h

@@ -2224,6 +2224,54 @@ GLFWAPI void glfwInitHint(int hint, int value);
  */
 GLFWAPI void glfwInitAllocator(const GLFWallocator* allocator);
 
+#if defined(VK_VERSION_1_0)
+
+/*! @brief Sets the desired Vulkan `vkGetInstanceProcAddr` function.
+ *
+ *  This function sets the `vkGetInstanceProcAddr` function that GLFW will use for all
+ *  Vulkan related entry point queries.
+ *
+ *  This feature is mostly useful on macOS, if your copy of the Vulkan loader is in
+ *  a location where GLFW cannot find it through dynamic loading, or if you are still
+ *  using the static library version of the loader.
+ *
+ *  If set to `NULL`, GLFW will try to load the Vulkan loader dynamically by its standard
+ *  name and get this function from there.  This is the default behavior.
+ *
+ *  The standard name of the loader is `vulkan-1.dll` on Windows, `libvulkan.so.1` on
+ *  Linux and other Unix-like systems and `libvulkan.1.dylib` on macOS.  If your code is
+ *  also loading it via these names then you probably don't need to use this function.
+ *
+ *  The function address you set is never reset by GLFW, but it only takes effect during
+ *  initialization.  Once GLFW has been initialized, any updates will be ignored until the
+ *  library is terminated and initialized again.
+ *
+ *  @param[in] loader The address of the function to use, or `NULL`.
+ *
+ *  @par Loader function signature
+ *  @code
+ *  PFN_vkVoidFunction vkGetInstanceProcAddr(VkInstance instance, const char* name)
+ *  @endcode
+ *  For more information about this function, see the
+ *  [Vulkan Registry](https://www.khronos.org/registry/vulkan/).
+ *
+ *  @errors None.
+ *
+ *  @remark This function may be called before @ref glfwInit.
+ *
+ *  @thread_safety This function must only be called from the main thread.
+ *
+ *  @sa @ref vulkan_loader
+ *  @sa @ref glfwInit
+ *
+ *  @since Added in version 3.4.
+ *
+ *  @ingroup init
+ */
+GLFWAPI void glfwInitVulkanLoader(PFN_vkGetInstanceProcAddr loader);
+
+#endif /*VK_VERSION_1_0*/
+
 /*! @brief Retrieves the version of the GLFW library.
  *
  *  This function retrieves the major, minor and revision numbers of the GLFW