Metal spot and defense range circles are now always drawn above the terrain but below the unit layer.

This fixes lines of circles sometimes being hidden inside terrain and a small issue with burrowed Fleas and other things sometimes being (partly) hidden below wide (high base metal) mex circles if people keep metal view on throughout the game.
Merge pull request #108 from Skasi/master

Draw metal spot and defense range circles between the terrain and the unit layer.