修改默认导出背景颜色为不透明黑色

Spine/Exporters/BaseExporter.cs

@@ -82,12 +82,12 @@ namespace Spine.Exporters
                 _backgroundColorPma = bcPma;
             }
         }
-        protected Color _backgroundColor = Color.Transparent;
+        protected Color _backgroundColor = Color.Black;
 
         /// <summary>
         /// 预乘后的背景颜色
         /// </summary>
-        protected Color _backgroundColorPma = Color.Transparent;
+        protected Color _backgroundColorPma = Color.Black;
 
         /// <summary>
         /// 画面分辨率

SpineViewer/ViewModels/Exporters/BaseExporterViewModel.cs

@@ -54,7 +54,7 @@ namespace SpineViewer.ViewModels.Exporters
         /// 背景颜色
         /// </summary>
         public Color BackgroundColor { get => _backgroundColor; set => SetProperty(ref _backgroundColor, value); }
-        protected Color _backgroundColor = Color.FromArgb(0, 0, 0, 0);
+        protected Color _backgroundColor = Color.FromArgb(255, 0, 0, 0);
 
         /// <summary>
         /// 四周边缘距离