wicdec: support alpha from WebP WIC decoder
wicdec: support alpha from WebP WIC decoder

the container check may not be strictly necessary now that the alpha
check has matured and the library will detect usable alpha, but it's
safer to fix this case first before making larger changes.

Bug: webp:533
Change-Id: I2e1ba42156970d579a52bd183707a037e65fd900

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