rewrite the bin_map clustering to use less memory
rewrite the bin_map clustering to use less memory

output should bit-write the same as before, in both
low_effort and non low_effort modes.

if anything, speed is a tad faster, probably because of the
reduced memory traffic.

Change-Id: Iaa2ddcfda2aaffefe7e5b7bc89216373d1ddb194

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