make the token page size be variable instead of fixed 8192

also changed the token-page layout a little bit to remove
a not-needed field.

This reduces the number of malloc()/free() calls substantially
with minimal increase in memory consumption (~2%).
For the tail of large sources, the number of malloc calls goes
typically from ~10000 to ~100 (e.g.: bryce_big.jpg: 22711 -> 105)

Change-Id: Ib847f41e618ed8c303d26b76da982fbc48de45b9

src/enc/webpenc.c

@@ -258,7 +258,12 @@ static VP8Encoder* InitVP8Encoder(const WebPConfig* const config,
   VP8EncInitLayer(enc);
 #endif
 
-  VP8TBufferInit(&enc->tokens_);
+  // lower quality means smaller output -> we modulate a little the page
+  // size based on quality. This is just a crude 1rst-order prediction.
+  {
+    const float scale = 1.f + config->quality * 5.f / 100.f;  // in [1,6]
+    VP8TBufferInit(&enc->tokens_, (int)(mb_w * mb_h * 4 * scale));
+  }
   return enc;
 }