Add utils to better quantize grayscale values to three bit indices while

src/utils/SkTextureCompressor_R11EAC.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkTextureCompressor.h"
 #include "SkTextureCompressor_Blitter.h"
+#include "SkTextureCompressor_Utils.h"
 
 #include "SkBlitter.h"
 #include "SkEndian.h"
 
 // #define COMPRESS_R11_EAC_SLOW 1
 // #define COMPRESS_R11_EAC_FAST 1
 #define COMPRESS_R11_EAC_FASTEST 1
 
 // Blocks compressed into R11 EAC are represented as follows:
 // 0000000000000000000000000000000000000000000000000000000000000000
@@ skipping 293 matching lines @@
 // To compute this, we first negate each byte, and then add three, which
 // gives the mapping
 // 3 2 1 0 -1 -2 -3 -4
 //
 // Then we mask out the negative values, take their absolute value, and
 // add three.
 //
 // Most of the voodoo in this function comes from Hacker's Delight, section 2-18
 static inline uint32_t convert_indices(uint32_t x) {
     // Take the top three bits...
-    x = (x & 0xE0E0E0E0) >> 5;
+    x = SkTextureCompressor::ConvertToThreeBitIndex(x);
 
     // Negate...
     x = ~((0x80808080 - x) ^ 0x7F7F7F7F);
 
     // Add three
     const uint32_t s = (x & 0x7F7F7F7F) + 0x03030303;
     x = ((x ^ 0x03030303) & 0x80808080) ^ s;
 
     // Absolute value
     const uint32_t a = x & 0x80808080;
@@ skipping 326 matching lines @@
     for (int j = 0; j < height; j += 4) {
         for (int i = 0; i < width; i += 4) {
             decompress_r11_eac_block(dst + i, dstRowBytes, src);
             src += 8;
         }
         dst += 4 * dstRowBytes;
     }    
 }
 
 }  // namespace SkTextureCompressor