cwebp: fix alpha reporting w/lossless & metadata
cwebp: fix alpha reporting w/lossless & metadata

the incorrect bit was being extracted from the lossless bitstream header
causing the alpha flag in VP8X to be misreported. previously the
signature byte was ignored in the calculation of the offset.

since:
8ba1bf61 Stricter check for presence of alpha when writing lossless
images

BUG=webp:361

Change-Id: I7c618b5f01a37f5e4b799dee11a7949efaf88046

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