允许导出单个时自动使用所有模型的所有动画最大时长

SpineViewer/ViewModels/Exporters/FFmpegVideoExporterViewModel.cs

@@ -51,7 +51,6 @@ namespace SpineViewer.ViewModels.Exporters
             using var exporter = new FFmpegVideoExporter(_renderer.Resolution.X + _margin * 2, _renderer.Resolution.Y + _margin * 2)
             {
                 BackgroundColor = new(_backgroundColor.R, _backgroundColor.G, _backgroundColor.B, _backgroundColor.A),
-                Duration = _duration,
                 Fps = _fps,
                 KeepLast = _keepLast,
                 Format = _format,
@@ -76,6 +75,7 @@ namespace SpineViewer.ViewModels.Exporters
                 var output = Path.Combine(_outputDir!, filename);
 
                 if (_autoResolution) SetAutoResolutionAnimated(exporter, spines);
+                if (_duration < 0) exporter.Duration = spines.Select(sp => sp.GetAnimationMaxDuration()).DefaultIfEmpty(0).Max();
 
                 exporter.ProgressReporter = (total, done, text) =>
                 {