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