Fix alpha channel and add inverted test

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -371,35 +371,36 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 padding = 4 - padding;
             }
 
-            byte[] row = new byte[arrayWidth + padding];
-            var color = default(TPixel);
-
-            var rgba = default(Rgba32);
-
-            for (int y = 0; y < height; y++)
+            using (var row = Buffer<byte>.CreateClean(arrayWidth + padding))
             {
-                int newY = Invert(y, height, inverted);
-                this.currentStream.Read(row, 0, row.Length);
-                int offset = 0;
-                Span<TPixel> pixelRow = pixels.GetRowSpan(newY);
+                var color = default(TPixel);
+                var rgba = new Rgba32(0, 0, 0, 255);
 
-                // TODO: Could use PixelOperations here!
-                for (int x = 0; x < arrayWidth; x++)
+                for (int y = 0; y < height; y++)
                 {
-                    int colOffset = x * ppb;
+                    int newY = Invert(y, height, inverted);
+                    this.currentStream.Read(row.Array, 0, row.Length);
+                    int offset = 0;
+                    Span<TPixel> pixelRow = pixels.GetRowSpan(newY);
 
-                    for (int shift = 0; shift < ppb && (x + shift) < width; shift++)
+                    // TODO: Could use PixelOperations here!
+                    for (int x = 0; x < arrayWidth; x++)
                     {
-                        int colorIndex = ((row[offset] >> (8 - bits - (shift * bits))) & mask) * 4;
-                        int newX = colOffset + shift;
+                        int colOffset = x * ppb;
 
-                        // Stored in b-> g-> r order.
-                        rgba.Bgr = Unsafe.As<byte, Bgr24>(ref colors[colorIndex]);
-                        color.PackFromRgba32(rgba);
-                        pixelRow[newX] = color;
-                    }
+                        for (int shift = 0; shift < ppb && (x + shift) < width; shift++)
+                        {
+                            int colorIndex = ((row[offset] >> (8 - bits - (shift * bits))) & mask) * 4;
+                            int newX = colOffset + shift;
 
-                    offset++;
+                            // Stored in b-> g-> r order.
+                            rgba.Bgr = Unsafe.As<byte, Bgr24>(ref colors[colorIndex]);
+                            color.PackFromRgba32(rgba);
+                            pixelRow[newX] = color;
+                        }
+
+                        offset++;
+                    }
                 }
             }
         }

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

tests/Images/Input/Bmp/test8-inverted.bmp

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:407c948e576330f603b3692c3d37248f8df31067f79e424dba6757066c97845e
+size 9270

tests/Images/Input/Bmp/test8.bmp

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:3551fbb385b6d7d54e2f0372f79e5701ca9215e4efdddb429ec5d7942367107f
+size 9270