Backends: Vulkan: Don't use VK_PRESENT_MODE_MAX_ENUM_KHR as specs state it isn't part of the API. (#5254)

backends/imgui_impl_vulkan.h

@@ -148,7 +148,7 @@ struct ImGui_ImplVulkanH_Window
     ImGui_ImplVulkanH_Window()
     {
         memset((void*)this, 0, sizeof(*this));
-        PresentMode = VK_PRESENT_MODE_MAX_ENUM_KHR;
+        PresentMode = (VkPresentModeKHR)~0;     // Ensure we get an error if user doesn't set this.
         ClearEnable = true;
     }
 };