fix

Author: Poker-sang

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -898,7 +898,7 @@ private void ReadRgbPaletteWithAlphaMask<TPixel>(BufferedReadStream stream, Buff
                     {
                         int colorIndex = ((rowSpan[offset] >> (8 - bitsPerPixel - (shift * bitsPerPixel))) & mask) * bytesPerColorMapEntry;
 
-                        image[newY, newX].FromBgr24(Unsafe.As<byte, Bgr24>(ref colors[colorIndex]));
+                        image[newY, newX] = Bgra32.FromBgr24(Unsafe.As<byte, Bgr24>(ref colors[colorIndex]));
                     }
 
                     offset++;
@@ -942,7 +942,7 @@ private void ReadRgbPaletteWithAlphaMask<TPixel>(BufferedReadStream stream, Buff
 
             for (int x = 0; x < width; x++)
             {
-                pixelRow[x].FromBgra32(image[newY, x]);
+                pixelRow[x] = TPixel.FromBgra32(image[newY, x]);
             }
         }
     }

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -792,8 +792,7 @@ private static void ProcessedAlphaMask<TPixel>(Stream stream, Image<TPixel> imag
     private static void WriteAlphaMask<TPixel>(in TPixel pixel, ref byte mask, in int index)
          where TPixel : unmanaged, IPixel<TPixel>
     {
-        Rgba32 rgba = default;
-        pixel.ToRgba32(ref rgba);
+        Rgba32 rgba = pixel.ToRgba32();
         if (rgba.A is 0)
         {
             mask |= unchecked((byte)(0b10000000 >> index));