Use push_back() for mtv to remove the slow O(n^2) algorithm.

This makes the sorting fly.  Still slow in AdjustRowHeight() which gets
called after the sort().  Will have to look at that later.

This change requires a newer version of mdds (not yet released).

Change-Id: I50c6ec9bb562af85c3b77201e2cd31b30dd719e9