multi-threaded segment analysis

When -mt is used, the analysis pass will be split in two
and each halves performed in parallel. This gives a 5%-9% speed-up.

This was a good occasion to revamp the iterator and analysis-loop
code. As a result, the default (non-mt) behaviour is a tad (~1%) faster.

Change-Id: Id0828c2ebe2e968db8ca227da80af591d6a4055f

src/enc/tree.c

@@ -344,7 +344,7 @@ void VP8CodeIntraModes(VP8Encoder* const enc) {
       }
     }
     PutUVMode(bw, mb->uv_mode_);
-  } while (VP8IteratorNext(&it, NULL));
+  } while (VP8IteratorNext(&it));
 }
 
 //------------------------------------------------------------------------------