CMS 9: Image creation fails due to window handle starvation

Description

After the CMS application runs for a while in Azure and generates large amounts of thumbnails, it stops generating scaled images (e.g., thumbnails). The below error message can be seen in the log files.

Exception calling ImageService RenderImage method (System.ComponentModel.Win32Exception (0x80004005): The operation completed successfully
at MS.Win32.UnsafeNativeMethods.RegisterClassEx(WNDCLASSEX_D wc_d)
at MS.Win32.HwndWrapper..ctor(Int32 classStyle, Int32 style, Int32 exStyle, Int32 x, Int32 y, Int32 width, Int32 height, String name, IntPtr parent, HwndWrapperHook[] hooks)
at System.Windows.Threading.Dispatcher..ctor()
at System.Windows.Threading.Dispatcher.get_CurrentDispatcher()
at System.Windows.Media.Imaging.BitmapSource..ctor(Boolean useVirtuals)
at EPiServer.ImageLibrary.ImageLib.LoadBitmapSource(Stream sourceStream, BitmapCreateOptions options)
at EPiServer.ImageLibrary.ImageService.RenderImage(Byte[] imageBuffer, IEnumerable`1 operations, String mimeType, Single zoomFactor, Int32 jpegQuality)
at EPiServer.ImageLibrary.ImageServiceClient.RenderImage(Byte[] imageBuffer, IEnumerable`1 operations, String mimeType, Single zoomFactor, Int32 jpegQuality)).