Corrected channels for the Fx methods.

Author: dlemstra

src/Magick.NET/MagickImage.cs

@@ -2859,7 +2859,7 @@ public void Frame(uint width, uint height, int innerBevel, int outerBevel)
     /// <param name="expression">The expression to apply.</param>
     /// <exception cref="MagickException">Thrown when an error is raised by ImageMagick.</exception>
     public void Fx(string expression)
-        => Fx(expression, ImageMagick.Channels.Undefined);
+        => Fx(expression, ImageMagick.Channels.All);
 
     /// <summary>
     /// Applies a mathematical expression to the image.

src/Magick.NET/MagickImageCollection.cs

@@ -558,7 +558,7 @@ public IMagickImage<QuantumType> Flatten(IMagickColor<QuantumType> backgroundCol
     /// <returns>The resulting image of the fx operation.</returns>
     /// <exception cref="MagickException">Thrown when an error is raised by ImageMagick.</exception>
     public IMagickImage<QuantumType> Fx(string expression)
-        => Fx(expression, Channels.Undefined);
+        => Fx(expression, Channels.All);
 
     /// <summary>
     /// Applies a mathematical expression to the images and returns the result.