get_disto: fix the r/g/b order for luma calculation
get_disto: fix the r/g/b order for luma calculation

Change-Id: I62be556f6254031e8c1f1a572cfdc4ea2641483b

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