Constantly re-allocating array is much slower than appending

[nikitakit]

The code at mesh-plugin.js:101 currently reads:

      // accumulate mesh vertices and uv
      var model = this.meshCustomBlock(value,x,y,z);
      vertices = vertices.concat(model.vertices);
      uv = uv.concat(model.uv);

The use of concat causes the allocation of new arrays (of ever increasing size), which is needlessly expensive. Appending in-place is much faster, e.g.

      var model = this.meshCustomBlock(value,x,y,z);
      for (var j = 0; j < model.vertices.length; j++) {
          vertices.push(model.vertices[j]);
      }
      for (var k = 0; k < model.uv.length; k++) {
          uv.push(model.uv[k]);
      }

I'm rendering some maps using a chunk distance of 2-4, 32x32x32 chunks, and using custom block models for a significant fraction of the blocks in the scene. This change significantly improves performance for my application (reducing meshing time by as much as 10 seconds!)