Render-order as map attribute, respected in actual in-editor rendering

As discussed here: https://github.com/bjorn/tiled/issues/455
In fact, even in right-down you have to do this. Just consider the theoretical case of a 1x1 map, where currently `startX` will be 0 and `endX` will be 0 too. Then `for (int y = startY; y != endY; y += incY) {` will do nothing. This is why in my code snippets I was doing `endX += incX` before the loops, to make sure the loop end condition would work correctly.