Allow the lossless cruncher to work for alpha.
Allow the lossless cruncher to work for alpha.

Change-Id: I7185e75404fae8c739e0536026f4687d25decad7

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