Backends: Vulkan: removed misleading code incrementing frameindex. (#7834)

Thanks NostraMagister!

backends/imgui_impl_vulkan.cpp

@@ -1820,7 +1820,7 @@ static void ImGui_ImplVulkan_RenderWindow(ImGuiViewport* viewport, void*)
         vd->SwapChainNeedRebuild = false;
     }
 
-    ImGui_ImplVulkanH_Frame* fd = &wd->Frames[wd->FrameIndex];
+    ImGui_ImplVulkanH_Frame* fd = nullptr;
     ImGui_ImplVulkanH_FrameSemaphores* fsd = &wd->FrameSemaphores[wd->SemaphoreIndex];
     {
         {
@@ -1983,8 +1983,6 @@ static void ImGui_ImplVulkan_SwapBuffers(ImGuiViewport* viewport, void*)
     {
         check_vk_result(err);
     }
-
-    wd->FrameIndex = (wd->FrameIndex + 1) % wd->ImageCount;             // This is for the next vkWaitForFences()
     wd->SemaphoreIndex = (wd->SemaphoreIndex + 1) % wd->SemaphoreCount; // Now we can use the next set of semaphores
 }