Add function for creating ASTC blitter

src/utils/SkTextureCompressor_Blitter.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTextureCompressor_Blitter_DEFINED
 #define SkTextureCompressor_Blitter_DEFINED
 
@@ skipping 213 matching lines @@
     }
 
     // Updates the block whose columns are stored in block. curAlphai is expected
     // to store the alpha values that will be placed within each of the columns in
     // the range [col, col+colsLeft).
     typedef uint32_t Column[BlockDim/4];
     typedef uint32_t Block[BlockDim][BlockDim/4];
     inline void updateBlockColumns(Block block, const int col,
                                    const int colsLeft, const Column curAlphai) {
         SkASSERT(NULL != block);
-        SkASSERT(col + colsLeft <= 4);
+        SkASSERT(col + colsLeft <= BlockDim);
 
         for (int i = col; i < (col + colsLeft); ++i) {
             memcpy(block[i], curAlphai, sizeof(Column));
         }
     }
 
     // The following function writes the buffered runs to compressed blocks.
     // If fNextRun < BlockDim, then we fill the runs that we haven't buffered with
     // the constant zero buffer.
     void flushRuns() {
@@ skipping 151 matching lines @@
             CompressionProc(outPtr, reinterpret_cast<uint8_t*>(block));
         }
 
         fNextRun = 0;
     }
 };
 
 }  // namespace SkTextureCompressor
 
 #endif  // SkTextureCompressor_Blitter_DEFINED