reduce memory usage by allocating only one histo
reduce memory usage by allocating only one histo

instead of lz77+rle

* introduce VP8LBackwardRefs structure and simplify the code by not passing
  around {PixOrCopy/int} pairs.

More functions should be turned into using this struct (TODO(later)).

Change-Id: I69c5c9fa61dddd61a2abc2824d70b8606a1c55b6

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