Fix 1 bit bmp decoding and add extra test images.

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -373,11 +373,9 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                     for (int x = 0; x < arrayWidth; x++)
                     {
                         int colOffset = x * ppb;
-
-                        for (int shift = 0; shift < ppb && (x + shift) < width; shift++)
+                        for (int shift = 0, newX = colOffset; shift < ppb && newX < width; shift++, newX++)
                         {
                             int colorIndex = ((rowSpan[offset] >> (8 - bits - (shift * bits))) & mask) * 4;
-                            int newX = colOffset + shift;
 
                             // Stored in b-> g-> r order.
                             rgba.Bgr = Unsafe.As<byte, Bgr24>(ref colors[colorIndex]);

tests/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs

@@ -17,10 +17,7 @@ namespace SixLabors.ImageSharp.Tests
     {
         public const PixelTypes CommonNonDefaultPixelTypes = PixelTypes.Rgba32 | PixelTypes.Bgra32 | PixelTypes.RgbaVector;
 
-        public static readonly string[] AllBmpFiles =
-        {
-            Car, F, NegHeight, CoreHeader, V5Header, RLE, RLEInverted, Bit8, Bit8Inverted, Bit16, Bit16Inverted
-        };
+        public static readonly string[] AllBmpFiles = All;
 
         public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
         new TheoryData<string, int, int, PixelResolutionUnit>

tests/ImageSharp.Tests/TestImages.cs

@@ -165,12 +165,30 @@ namespace SixLabors.ImageSharp.Tests
             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 Bit1 = "Bmp/pal1.bmp";
+            public const string Bit1Pal1 = "Bmp/pal1p1.bmp";
+            public const string Bit4 = "Bmp/pal4.bmp";
             public const string Bit8 = "Bmp/test8.bmp";
             public const string Bit8Inverted = "Bmp/test8-inverted.bmp";
             public const string Bit16 = "Bmp/test16.bmp";
             public const string Bit16Inverted = "Bmp/test16-inverted.bmp";
 
-            public static readonly string[] All = { Car, F, NegHeight, CoreHeader, V5Header, RLE, RLEInverted, Bit8, Bit8Inverted, Bit16, Bit16Inverted };
+            public static readonly string[] All
+            = {
+                Car,
+                F,
+                NegHeight,
+                CoreHeader,
+                V5Header, RLE,
+                RLEInverted,
+                Bit1,
+                Bit1Pal1,
+                Bit4,
+                Bit8,
+                Bit8Inverted,
+                Bit16,
+                Bit16Inverted
+            };
         }
 
         public static class Gif