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

src/init.c

@@ -55,6 +55,7 @@ static _GLFWinitconfig _glfwInitHints =
     GLFW_TRUE,      // hat buttons
     GLFW_ANGLE_PLATFORM_TYPE_NONE, // ANGLE backend
     GLFW_ANY_PLATFORM, // preferred platform
+    NULL,           // vkGetInstanceProcAddr function
     {
         GLFW_TRUE,  // macOS menu bar
         GLFW_TRUE   // macOS bundle chdir
@@ -404,6 +405,11 @@ GLFWAPI void glfwInitAllocator(const GLFWallocator* allocator)
         memset(&_glfwInitAllocator, 0, sizeof(GLFWallocator));
 }
 
+GLFWAPI void glfwInitVulkanLoader(PFN_vkGetInstanceProcAddr loader)
+{
+    _glfwInitHints.vulkanLoader = loader;
+}
+
 GLFWAPI void glfwGetVersion(int* major, int* minor, int* rev)
 {
     if (major != NULL)