Add no-color cache configuration to the cruncher

Fix another pessimization found by the pingo image compressor.

Refactoring is necessary to make LZ77 computation
common to cache or no-cache analysis.
Slower by 1.7x instead of 2x

Change-Id: I396701ea6e88543dbfe9471eb552877f6c8ce1e3

src/enc/histogram_enc.c

@@ -208,6 +208,7 @@ void VP8LHistogramAddSinglePixOrCopy(VP8LHistogram* const histo,
   } else if (PixOrCopyIsCacheIdx(v)) {
     const int literal_ix =
         NUM_LITERAL_CODES + NUM_LENGTH_CODES + PixOrCopyCacheIdx(v);
+    assert(histo->palette_code_bits_ != 0);
     ++histo->literal_[literal_ix];
   } else {
     int code, extra_bits;