Treat YU12 as an alias for I420. Simplify setting of inv_crop_height.

source/convert_from.cc

 /*
  *  Copyright 2012 The LibYuv Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS. All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 1059 matching lines @@
                      dst_sample_stride ? dst_sample_stride : width,
                      dst_vu,
                      dst_sample_stride ? dst_sample_stride : width,
                      width, height);
       break;
     }
     // TODO(fbarchard): Add M420.
     // Triplanar formats
     // TODO(fbarchard): halfstride instead of halfwidth
     case FOURCC_I420:
-    case FOURCC_YU12:
     case FOURCC_YV12: {
       int halfwidth = (width + 1) / 2;
       int halfheight = (height + 1) / 2;
       uint8* dst_u;
       uint8* dst_v;
       if (format == FOURCC_YV12) {
         dst_v = dst_sample + width * height;
         dst_u = dst_v + halfwidth * halfheight;
       } else {
         dst_u = dst_sample + width * height;
@@ skipping 67 matching lines @@
     default:
       return -1;  // unknown fourcc - return failure code.
   }
   return r;
 }
 
 #ifdef __cplusplus
 }  // extern "C"
 }  // namespace libyuv
 #endif