webpinfo: add support to parse Alpha bitstream
webpinfo: add support to parse Alpha bitstream

BUG=webp:330

Change-Id: I9ed0923d19558052d63bdbe93a542c51595fdce7

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