_setMapDiv adjustments

Fixes #38

The issue I noticed was when a page had more body content bellow the map. This caused the _setMapDiv calculation to get a negative or rapidly fluctuating value for mh2. The result was the map rapidly flashed from small to large and often became stuck at a small size. This calculation takes the min of the previous window size and body content, or just takes body content if old window is 0 (not initialized). This solved all of my flickering issues.