ConvertWRGBToYUV: use relative pointer offsets
ConvertWRGBToYUV: use relative pointer offsets

avoids int rollover when working with large input

BUG=webp:312

Change-Id: I693cbb295df9cf94aa89294b19c0496bdbe84d18

PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp