Add GLFW_X11_XCB_VULKAN_SURFACE init hint

This hint controls whether GLFW will try to use VK_KHR_xcb_surface
before falling back to VK_KHR_xlib_surface.

Closes #1793.

src/internal.h

@@ -248,6 +248,9 @@ struct _GLFWinitconfig
         GLFWbool  menubar;
         GLFWbool  chdir;
     } ns;
+    struct {
+        GLFWbool  xcbVulkanSurface;
+    } x11;
 };
 
 // Window configuration