fix potential overflow when width * height * 4 >= (1<<32)
fix potential overflow when width * height * 4 >= (1<<32)

Mostly: avoid doing calculation like: ptr + j * stride
when stride is 'int'. Rather use size_t, or pointer increments (ptr += stride)
when possible.

BUG=webp:314

Change-Id: I81c684b515dd1ec4f601f32d50a6e821c4e46e20

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