new segmentation algorithm

fixes the 'blocky sky problem' (saturation problem: when luma was flat,
chroma noise was taking over, resulting in random segment id assigned.
When just using a common uniform segment was better).

+ side clean-up and readibility/experimentability MACRO'ization
+ added '-map 7' option

Change-Id: I35982a9e43c0fecbfdd7b05e4813e8ba8c121d71

src/enc/frame.c

@@ -736,6 +736,7 @@ static void StoreSideInfo(const VP8EncIterator* const it) {
         const int b = (int)((it->luma_bits_ + it->uv_bits_ + 7) >> 3);
         *info = (b > 255) ? 255 : b; break;
       }
+      case 7: *info = mb->alpha_; break;
       default: *info = 0; break;
     };
   }