Make image decoders faster by refactoring hot loops that fills the lines of ImageFrame.

Source/wtf/MathExtras.h

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 416 matching lines @@
         log2 += 8, i >>= 8;
     if (i >> 4)
         log2 += 4, i >>= 4;
     if (i >> 2)
         log2 += 2, i >>= 2;
     if (i >> 1)
         log2 += 1;
     return log2;
 }
 
+inline uint16_t fastDivideBy255(uint16_t value)
+{
+    // This is an approximate algorithm for division by 255, but it gives accurate results for 16bit values.
+    uint16_t approximation = value >> 8;
+    uint16_t remainder = value - (approximation * 255) + 1;
+    return approximation + (remainder >> 8);
+}
+
 } // namespace WTF
 
 #endif // #ifndef WTF_MathExtras_h