Add function for creating ASTC blitter

src/utils/SkTextureCompressor_Blitter.h

Index: src/utils/SkTextureCompressor_Blitter.h
diff --git a/src/utils/SkTextureCompressor_Blitter.h b/src/utils/SkTextureCompressor_Blitter.h
index 23265a4d184c6645d72c6ea38544f3d75730d57d..93d158dad8d646e17015a9040a8f9789b83fb45e 100644
--- a/src/utils/SkTextureCompressor_Blitter.h
+++ b/src/utils/SkTextureCompressor_Blitter.h
@@ -231,7 +231,7 @@ private:
     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));