[ImageExport] Synchronous invokation deadlock still occures

Problem

This is the problem that we intended to fix with PR #321. It is fixed partially.

After we migrated to tasks instead of F#'s async, plus we changed

- page.CloseAsync() |> Async.AwaitTask |> Async.RunSynchronously
+ page.CloseAsync() |> AsyncHelper.taskSyncUnit

The async version works, as in, completes. However, the synchronous version still does not terminate

Reproduction

I'm going to present the repro with AvaloniaUI, but it should be as well reproduced in WinForms or WPF if you're on Windows.

1. Install templates & create project:

dotnet new --install Avalonia.Templates
dotnet new avalonia.mvvm -o TestAsync -n TestAsync
cd TestAsync/

2. Install the package

Add source & install the latest

dotnet new nugetconfig
dotnet nuget add source https://www.myget.org/F/plotly-net-nightly/api/v3/index.json
dotnet add package Plotly.NET.ImageExport --prerelease

3. Adding a component

Go to MainWindow.axaml and replace some default thingy with a button

- <TextBlock Text="{Binding Greeting}" HorizontalAlignment="Center" VerticalAlignment="Center"/>
+ <Button Click="Button_OnClick" />

4. Adding event

Go to MainWindow.axaml.cs and add this method:

        private async void Button_OnClick(object? sender, RoutedEventArgs e)
        {
            var layout = new Layout();
            var trace = new Trace("bar");
            var fig = GenericChart.Figure.create(ListModule.OfSeq(new[] { trace }), layout);
           
            var chart = GenericChart.fromFigure(fig);


            // This works now
            await chart.ToBase64PNGStringAsync();

            // This never terminates
            chart.ToBase64PNGString();
            
            // If we get to this line, it terminated
            throw new Exception();
        }

5. Run

dotnet run

Now, once it started, click the button. Successful completion is when the exception occurs.

从 ImageExport 的同步路径开始，尤其是 page.CloseAsync() 和 AsyncHelper.taskSyncUnit，并将其与正常工作的 ToBase64PNGStringAsync() 路径进行比较。使用报告中所示的 Avalonia 按钮处理程序重现该问题；当 chart.ToBase64PNGString() 终止并到达以下异常时，即完成。