#771: change order of addition as proposed by @dlemstra

added comments as explanation

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -310,8 +310,8 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 }
                 else
                 {
-                    int max = cmd[0] + count;
-                    byte cmd1 = cmd[1];
+                    int max = count + cmd[0]; // as we start at the current count in the following loop, max is count + cmd[0]
+                    byte cmd1 = cmd[1]; // store the value to avoid the repeated indexer access inside the loop
 
                     for (; count < max; count++)
                     {