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/init.c

@@ -57,7 +57,10 @@ static _GLFWinitconfig _glfwInitHints =
     {
         GLFW_TRUE,  // macOS menu bar
         GLFW_TRUE   // macOS bundle chdir
-    }
+    },
+    {
+        GLFW_TRUE,  // X11 XCB Vulkan surface
+    },
 };
 
 // Terminate the library
@@ -298,6 +301,9 @@ GLFWAPI void glfwInitHint(int hint, int value)
         case GLFW_COCOA_MENUBAR:
             _glfwInitHints.ns.menubar = value;
             return;
+        case GLFW_X11_XCB_VULKAN_SURFACE:
+            _glfwInitHints.x11.xcbVulkanSurface = value;
+            return;
     }
 
     _glfwInputError(GLFW_INVALID_ENUM,