Fix bug in black and white plain decoding

src/ImageSharp/Formats/Pbm/PlainDecoder.cs

@@ -174,8 +174,8 @@ namespace SixLabors.ImageSharp.Formats.Pbm
             MemoryAllocator allocator = configuration.MemoryAllocator;
             using IMemoryOwner<L8> row = allocator.Allocate<L8>(width);
             Span<L8> rowSpan = row.GetSpan();
-            var white = new L8(0);
-            var black = new L8(255);
+            var white = new L8(255);
+            var black = new L8(0);
 
             for (int y = 0; y < height; y++)
             {

src/ImageSharp/Formats/Pbm/PlainEncoder.cs

@@ -230,7 +230,7 @@ namespace SixLabors.ImageSharp.Formats.Pbm
                 int written = 0;
                 for (int x = 0; x < width; x++)
                 {
-                    byte value = (rowSpan[x].PackedValue > 127) ? Zero : One;
+                    byte value = (rowSpan[x].PackedValue < 128) ? One : Zero;
                     plainSpan[written++] = value;
                     plainSpan[written++] = Space;
                 }

tests/ImageSharp.Tests/Formats/Pbm/PbmRoundTripTests.cs

@@ -14,6 +14,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Pbm
     public class PbmRoundTripTests
     {
         [Theory]
+        [InlineData(BlackAndWhitePlain)]
         [InlineData(BlackAndWhiteBinary)]
         [InlineData(GrayscalePlain)]
         [InlineData(GrayscaleBinary)]