Tune BackwardReferencesLz77 for low_effort (m=0).

- Lower the threshold parameters for HashChainFindCopy.

For 1000 image PNG corpus (m=0), this change yields speedup of 15-20% at
lower quality range (0.25% drop in compression density) and about 10%
for higher quality range without any drop in the compression density.
Following is the compression stats (before/after) for method = 0:
         Before           After
         bpp/MPs          bpp/MPs
q=0      2.8615/18.000    2.8651/18.631
q=5      2.8615/18.216    2.8650/20.517
q=10     2.8572/18.070    2.8650/21.992
q=15     2.8519/18.371    2.8584/21.747
q=20     2.8454/18.975    2.8515/20.448
q=25     2.8230/8.531     2.8253/9.585
// Compression density remains same for q-range [30-100]
q=30     2.7310/7.706     2.7310/8.028
q=35     2.7253/6.855     2.7253/7.184
q=40     2.7231/6.364     2.7231/6.604
q=45     2.7216/5.844     2.7216/6.223
q=50     2.7196/5.210     2.7196/5.731
q=55     2.7208/4.766     2.7208/4.970
q=60     2.7195/4.495     2.7195/4.602
q=65     2.7185/4.024     2.7185/4.236
q=70     2.7174/3.699     2.7174/3.861
q=75     2.7164/3.449     2.7164/3.605
q=80     2.7161/3.222     2.7161/3.038
q=85     2.7153/2.919     2.7153/2.946
q=90     2.7145/2.766     2.7145/2.771
q=95     2.7124/2.548     2.7124/2.575
q=100    2.6873/2.253     2.6873/2.335

Change-Id: I0e17581fb71f6094032ad06c6203350bd502f9a1

src/enc/backward_references.h

@@ -192,7 +192,7 @@ static WEBP_INLINE void VP8LRefsCursorNext(VP8LRefsCursor* const c) {
 // refs[0] or refs[1].
 VP8LBackwardRefs* VP8LGetBackwardReferences(
     int width, int height, const uint32_t* const argb, int quality,
-    int* cache_bits, VP8LHashChain* const hash_chain,
+    int low_effort, int* cache_bits, VP8LHashChain* const hash_chain,
     VP8LBackwardRefs refs[2]);
 
 #ifdef __cplusplus