Fix //snowsmooth not working below Y0 (#1769)

* Use min y instead of 0 for SnowHeightMap

* Correctly handle rounding for height

* Revert old fix

* Use y instead of newHeight

* Remove line diff

worldedit-core/src/main/java/com/sk89q/worldedit/math/convolution/SnowHeightMap.java

@@ -232,7 +232,7 @@ public class SnowHeightMap {
         int y = (int) Math.floor(newHeight);
         int numOfLayers = (int) ((newHeight - y) * 8) + 1;
         //FAWE start - avoid BlockVector3 creation for no reason
-        session.setBlock(x, (int) newHeight, z, BlockTypes.SNOW.getDefaultState().with(LAYERS, numOfLayers));
+        session.setBlock(x, y, z, BlockTypes.SNOW.getDefaultState().with(LAYERS, numOfLayers));
         //FAWE end
     }