Add RLE inverted test image

tests/ImageSharp.Tests/TestImages.cs

@@ -129,16 +129,19 @@ namespace SixLabors.ImageSharp.Tests
 
         public static class Bmp
         {
+            // Note: The inverted images have been generated by altering the BitmapInfoHeader using a hex editor.
+            // As such, the expected pixel output will be the reverse of the unaltered equivalent images.
             public const string Car = "Bmp/Car.bmp";
             public const string F = "Bmp/F.bmp";
             public const string NegHeight = "Bmp/neg_height.bmp";
             public const string CoreHeader = "Bmp/BitmapCoreHeaderQR.bmp";
             public const string V5Header = "Bmp/BITMAPV5HEADER.bmp";
             public const string RLE = "Bmp/RunLengthEncoded.bmp";
+            public const string RLEInverted = "Bmp/RunLengthEncoded-inverted.bmp";
             public const string Bit8 = "Bmp/test8.bmp";
             public const string Bit8Inverted = "Bmp/test8-inverted.bmp";
 
-            public static readonly string[] All = { Car, F, NegHeight, CoreHeader, V5Header, RLE, Bit8, Bit8Inverted };
+            public static readonly string[] All = { Car, F, NegHeight, CoreHeader, V5Header, RLE, RLEInverted, Bit8, Bit8Inverted };
         }
 
         public static class Gif