Simplify collision boxes

The collison box of stones is by default the same
shape as the nodebox, not the selection box.
When a player is walking across the top of a bunch
of stones with the more complex collision box
based on the node box, it causes some judder
due to the player fitting into some small contours
and "falling" a pixel or two before climbing back
out.  Making the collision boxes match the simpler
selection boxes prevents this.

stones.lua

@@ -47,6 +47,10 @@ local function reg(name, basename, basedef)
             type = "fixed",
             fixed = {-7/16, -8/16, -7/16, 7/16, 2/16, 7/16}
         },
+        collision_box = {
+            type = "fixed",
+            fixed = {-7/16, -8/16, -7/16, 7/16, 2/16, 7/16}
+        },
 
 
         tiles = {
@@ -91,7 +95,10 @@ local function reg(name, basename, basedef)
             type = "fixed",
             fixed = {-6/16, -8/16, -6/16, 6/16, 0/16, 6/16}
         },
-
+        collision_box = {
+            type = "fixed",
+            fixed = {-6/16, -8/16, -6/16, 6/16, 0/16, 6/16}
+        },
 
         tiles = {
             basedef.tiles[1]