anim_decode: fix alpha blending with big-endian
anim_decode: fix alpha blending with big-endian

images are decoded in RGBA/BGRA, but represented as uint32_t during the
blend process; this fixes the channel extraction

Bug: webp:548
Change-Id: Ie74aa43d8f87d3552d5afc0abba466335f5d1617

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