Speed up BackwardReferences

Speed up BackwardReferencesHashChainDistanceOnly method by:
1.) Remove for loop for shortmax code path.
2.) Execute the shortmax code path after regular call to
HashChainFindCopy, only if HashChainFindCopy() returns length > 2 (MIN_LENGTH).
3.) Also for shortmax, call method HashChainFindOffset (for length = 2),
instead of expensive method HashChainFindCopy().
4.) Handling first pixel (i==0) outside main loop and removing one if
condition (i > 0) per pixel.
5.) Handle the last pixel outside the main 'for' loop.

Overall compression speedup observed is around 5% (+/- noise).

Change-Id: Ifa30c4035f8d26e6e43e3c4881244d777961c22b
gif2webp: Add '-min-size' option to get best compression.

This disables key-frame insertion and tries both dispose methods for
each frame.

Change-Id: Ib167747198fd1d98cb93321f76826e91228f24d8