Fix build error

src/ImageSharp/Formats/Bmp/BmpInfoHeader.cs

@@ -66,6 +66,16 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// </summary>
         public const int HeaderSizeSize = 4;
 
+        /// <summary>
+        /// Maximum dimensions of a bitmap with or height: 2**31 - 1, since width and height are int32
+        /// </summary>
+        private const int MaximumBmpDimension = 2147483647;
+
+        /// <summary>
+        /// Maximum size of a bitmap width * height: 2**32, since size is uint32.
+        /// </summary>
+        private const long MaximumBmpSize = 4294967296;
+
         public BmpInfoHeader(
             int headerSize,
             int width,
@@ -545,8 +555,6 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
         internal void VerifyDimensions()
         {
-            const int MaximumBmpDimension = 2147483647; // 2**31 - 1, since width & height are int32
-
             if (this.Width > MaximumBmpDimension || this.Height > MaximumBmpDimension)
             {
                 throw new InvalidOperationException(
@@ -554,8 +562,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                     + $"bigger then the max allowed '{MaximumBmpDimension}'");
             }
 
-            const long MaximumBmpSize = 4294967296; // 2**32, since size is uint32
-            const long size = (long)this.Width * (long)this.Height;
+            long size = this.Width * this.Height;
             if (size > MaximumBmpSize)
             {
                 throw new InvalidOperationException(