fix multithread related crashes in skpdiff

tools/skpdiff/SkCLImageDiffer.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <cstring>
 
 #include "SkBitmap.h"
@@ skipping 66 matching lines @@
     cl_int kernelErr;
     *kernel = clCreateKernel(program, name, &kernelErr);
     if (CL_SUCCESS != kernelErr) {
         SkDebugf("Kernel creation failed: %s\n", cl_error_to_string(kernelErr));
         return false;
     }
 
     return true;
 }
 
-bool SkCLImageDiffer::makeImage2D(SkBitmap* bitmap, cl_mem* image) {
+bool SkCLImageDiffer::makeImage2D(SkBitmap* bitmap, cl_mem* image) const {
     cl_int imageErr;
     cl_image_format bitmapFormat;
     switch (bitmap->config()) {
         case SkBitmap::kA8_Config:
             bitmapFormat.image_channel_order = CL_A;
             bitmapFormat.image_channel_data_type = CL_UNSIGNED_INT8;
             break;
         case SkBitmap::kRGB_565_Config:
             bitmapFormat.image_channel_order = CL_RGB;
             bitmapFormat.image_channel_data_type = CL_UNORM_SHORT_565;
@@ skipping 15 matching lines @@
                              &imageErr);
     bitmap->unlockPixels();
 
     if (CL_SUCCESS != imageErr) {
         SkDebugf("Input image creation failed: %s\n", cl_error_to_string(imageErr));
         return false;
     }
 
     return true;
 }