add lossless incremental decoding support

* We don't need to change DecodeAlpha, since incremental
decoding is not useful for Alpha (we already decode
progressively along the RGB)
* Similarly, we don't do incremental decoding for level>0 planes:
   the metadata don't turn into visible pixel (only the ones in level0), so...
(No visible speed change)

Change-Id: I2fd4b9ba227561a7dbede647686584b752be7baa

src/utils/color_cache.c

@@ -13,6 +13,7 @@
 
 #include <assert.h>
 #include <stdlib.h>
+#include <string.h>
 #include "./color_cache.h"
 #include "../utils/utils.h"
 
@@ -27,6 +28,7 @@ int VP8LColorCacheInit(VP8LColorCache* const cc, int hash_bits) {
                                           sizeof(*cc->colors_));
   if (cc->colors_ == NULL) return 0;
   cc->hash_shift_ = 32 - hash_bits;
+  cc->hash_bits_ = hash_bits;
   return 1;
 }
 
@@ -37,3 +39,11 @@ void VP8LColorCacheClear(VP8LColorCache* const cc) {
   }
 }
 
+void VP8LColorCacheCopy(const VP8LColorCache* const src,
+                        VP8LColorCache* const dst) {
+  assert(src != NULL);
+  assert(dst != NULL);
+  assert(src->hash_bits_ == dst->hash_bits_);
+  memcpy(dst->colors_, src->colors_,
+         (1 << dst->hash_bits_) * sizeof(*dst->colors_));
+}