Add altitude adjustment for terrain image. Fix bug.

Added an adjustment for the terrain image, so that if its horizontal center is not at 0 degrees, it can be offset without taking another picture. This was requested by a couple folks on the forum, and was straight-forward to implement. https://indilib.org/forum/ekos/9035-new-feature-terrain-backgrounds.html?start=36#74427 https://indilib.org/forum/ekos/9035-new-feature-terrain-backgrounds.html?start=0#68563

Also cleaned up some dead code, and noticed a bug that may have caused excessive computation: On this line: https://invent.kde.org/education/kstars/-/blob/master/kstars/terrain/terrainrenderer.cpp#L389 = should be !=