Fix alpha channel and add inverted test

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -371,15 +371,15 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 padding = 4 - padding;
             }
 
-            byte[] row = new byte[arrayWidth + padding];
+            using (var row = Buffer<byte>.CreateClean(arrayWidth + padding))
+            {
                 var color = default(TPixel);
-
+                var rgba = new Rgba32(0, 0, 0, 255);
-            var rgba = default(Rgba32);
 
                 for (int y = 0; y < height; y++)
                 {
                     int newY = Invert(y, height, inverted);
-                this.currentStream.Read(row, 0, row.Length);
+                    this.currentStream.Read(row.Array, 0, row.Length);
                     int offset = 0;
                     Span<TPixel> pixelRow = pixels.GetRowSpan(newY);
 
@@ -403,6 +403,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                     }
                 }
             }
+        }
 
         /// <summary>
         /// Reads the 16 bit color palette from the stream

tests/ImageSharp.Tests/TestImages.cs

@@ -135,8 +135,10 @@ namespace SixLabors.ImageSharp.Tests
             public const string CoreHeader = "Bmp/BitmapCoreHeaderQR.bmp";
             public const string V5Header = "Bmp/BITMAPV5HEADER.bmp";
             public const string RLE = "Bmp/RunLengthEncoded.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 };
+            public static readonly string[] All = { Car, F, NegHeight, CoreHeader, V5Header, RLE, Bit8, Bit8Inverted };
         }
 
         public static class Gif