Import: use relative pointer offsets
Import: use relative pointer offsets

avoids int rollover when working with large input

BUG=webp:312

Change-Id: I6ad9f93b6c4b665c559bff87716a7b847f66a20d

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