Added internal accessor for root frame

src/ImageSharp/ImageFrameCollection{TPixel}.cs

@@ -79,6 +79,15 @@ namespace SixLabors.ImageSharp
             }
         }
 
+        /// <summary>
+        /// Gets root frame accessor in unsafe manner without any checks.
+        /// </summary>
+        /// <remarks>
+        /// This property is most likely to be called from <see cref="Image{TPixel}"/> for indexing pixels.
+        /// <see cref="Image{TPixel}"/> already checks if it was disposed before querying for root frame.
+        /// </remarks>
+        internal ImageFrame<TPixel> RootFrameUnsafe => this.frames[0];
+
         /// <inheritdoc />
         protected override ImageFrame NonGenericRootFrame => this.RootFrame;
 

src/ImageSharp/Image{TPixel}.cs

@@ -158,7 +158,7 @@ namespace SixLabors.ImageSharp
         /// <summary>
         /// Gets the root frame.
         /// </summary>
-        private IPixelSource<TPixel> PixelSource => this.frames.RootFrame;
+        private IPixelSource<TPixel> PixelSource => this.frames.RootFrameUnsafe;
 
         /// <summary>
         /// Gets or sets the pixel at the specified position.