Android420ToI420 - use ptrdiff_t for difference of u and v pointers

The difference was assigned to an int, causing a warning on Visual C.

BUG=662
TEST=tested with try bots.
R=devangelakos@google.com

Review-Url: https://codereview.chromium.org/2574373002 .

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 1636
+Version: 1637
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 1636
+#define LIBYUV_VERSION 1637
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_

source/convert.cc

@@ -1496,7 +1496,7 @@ int Android420ToI420(const uint8* src_y,
                      int width,
                      int height) {
   int y;
-  const int vu_off = src_v - src_u;
+  const ptrdiff_t vu_off = src_v - src_u;
   int halfwidth = (width + 1) >> 1;
   int halfheight = (height + 1) >> 1;
   if (!src_u || !src_v || !dst_u || !dst_v || width <= 0 || height == 0) {