Viewport: Removed back-end renderer code that implied we need to clear with WindowBg color. Added NoRendererClear flag. (#1542, #1042)

examples/imgui_impl_vulkan.h

@@ -77,6 +77,7 @@ struct ImGui_ImplVulkan_WindowData
     VkSurfaceFormatKHR  SurfaceFormat;
     VkPresentModeKHR    PresentMode;
     VkRenderPass        RenderPass;
+    bool                ClearEnable;
     VkClearValue        ClearValue;
     uint32_t            BackBufferCount;
     VkImage             BackBuffer[16];