Optimize BackwardReferences for RLE encoding.

Updated BackwardReferencesRle method by utilizing the local color cache.
Also changed the name of method BackwardReferencesHashChain to
BackwardReferencesLz77 to reflect the LZ77 coding.

For the 1000 image corpus, this change saves 0.2% bytes
(at default settings) and is 2-5% faster to encode.

Change-Id: Ic3f288253b3bbb101a69945a80994c3fd0917f8b
backward_references.c: reindent after c8581b0

Change-Id: Icfc0fe8e266c0f67a70b8cb095e5aaee155290b6