webpinfo: correct background color output, BGRA->ARGB
webpinfo: correct background color output, BGRA->ARGB

bgcolor is stored little-endian, byte order is BGRA, with A being the
high byte

Change-Id: I8327380c039634120ee2313044781a5651513dcb

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