Importing terrain heightmaps flips Y-axis

This bug has been around for ages, most recently referred to [here](http://www.garagegames.com/community/forums/viewthread/107197). I fixed it in the CEV, and I've now ported my work to MIT. This time, though, the flipping is controlled by the last parameter passed to TerrainBlock::import so you could control it from the editor with a checkbox.

I don't want to add a checkbox to all four template projects' editor files. So someone else can do that, or I'll wait until the templates are refactored.