Reduce memory footprint for encoding WebP lossless.

Reduce calls to Malloc (WebPSafeMalloc/WebPSafeCalloc) for:
- Building HashChain data-structure used in creating the backward references.
- Creating Backward references for LZ77 or RLE coding.
- Creating Huffman tree for encoding the image.
For the above mentioned code-paths, allocate memory once and re-use it
subsequently.

Reduce the foorprint of VP8LHistogram struct by changing the Struct
field 'literal_' from an array of constant size to dynamically allocated
buffer based on the input parameter cache_bits.

Initialize BitWriter buffer corresponding to 16bpp (2*W*H).
There are some hard-files that are compressed at 12 bpp or more. The
realloc is costly and can be avoided for most of the WebP lossless
images by allocating some extra memory at the encoder initializaiton.

Change-Id: I1ea8cf60df727b8eb41547901f376c9a585e6095

src/enc/vp8li.h

@@ -23,6 +23,8 @@
 extern "C" {
 #endif
 
+struct VP8LHashChain;  // Defined in backward_references.h
+
 typedef struct {
   const WebPConfig* config_;    // user configuration and parameters
   const WebPPicture* pic_;      // input picture.
@@ -33,6 +35,11 @@ typedef struct {
   uint32_t* transform_data_;    // Scratch memory for transform data.
   int       current_width_;     // Corresponds to packed image width.
 
+  struct VP8LHashChain* hash_chain_;  // HashChain data for constructing
+                                      // backward references.
+  struct VP8LBackwardRefs* refs_[2];  // Backward Refs array corresponding to
+                                      // LZ77 & RLE coding.
+
   // Encoding parameters derived from quality parameter.
   int histo_bits_;
   int transform_bits_;