Terrain deformation loops scan from row 0 wastefully

[bobtista]

createCraterInTerrain and all four flattenTerrain loops calculate iMin.y, then ignore it and start at j = 0. The shape checks reject those rows, so the terrain result is correct, but the wasted work increases with the object's Y position.

Reproduction:

1. Spawn BlastCraterGenerator at a high Y coordinate.
2. Profile createCraterInTerrain.
3. It scans each column from row 0 instead of iMin.y.