remove conversion U32 -> S8 warnings
remove conversion U32 -> S8 warnings

using an inline U32ToS8() function

Change-Id: I45f535c6c9b5de33d69acc17b466e183fcc19a63

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