Don't use RGB. Fixes #1275

Author: JimBobSquarePants

src/ImageSharp/PixelFormats/PixelImplementations/Rgb24.cs

@@ -201,7 +201,18 @@ public void FromLa32(La32 source)
 
         /// <inheritdoc/>
         [MethodImpl(InliningOptions.ShortMethod)]
-        public void FromRgba32(Rgba32 source) => this = source.Rgb;
+        public void FromRgba32(Rgba32 source)
+        {
+#if NETSTANDARD2_0
+
+            // See https://github.com/SixLabors/ImageSharp/issues/1275
+            this.R = source.R;
+            this.G = source.G;
+            this.B = source.B;
+#else
+            this = source.Rgb;
+#endif
+        }
 
         /// <inheritdoc />
         [MethodImpl(InliningOptions.ShortMethod)]