Expose CurrentOpacity on ISkiaDrawingContextImpl.

src/Skia/Avalonia.Skia/DrawingContextImpl.cs

@@ -126,6 +126,7 @@ namespace Avalonia.Skia
         SKCanvas ISkiaDrawingContextImpl.SkCanvas => Canvas;
         SKSurface ISkiaDrawingContextImpl.SkSurface => Surface;
         GRContext ISkiaDrawingContextImpl.GrContext => _grContext;
+        double ISkiaDrawingContextImpl.CurrentOpacity => _currentOpacity;
 
         /// <inheritdoc />
         public void Clear(Color color)

src/Skia/Avalonia.Skia/ISkiaDrawingContextImpl.cs

@@ -8,5 +8,6 @@ namespace Avalonia.Skia
         SKCanvas SkCanvas { get; }
         GRContext GrContext { get; }
         SKSurface SkSurface { get; }
+        double CurrentOpacity { get; }
     }
 }