Fix regression from 2.3 with destroy(platforms)

2.3 introduced a regression with destroy(platforms). The problem was
that isplatform wasn't being set to false when the entity got disabled,
so if the platform was moving, it would keep moving until it hit a wall,
instead of stopping immediately.

desktop_version/src/Entity.cpp

@@ -1106,6 +1106,7 @@ bool entityclass::disableentity(int t)
     entities[t].size = -1;
     entities[t].type = -1;
     entities[t].rule = -1;
+    entities[t].isplatform = false;
 
     return true;
 }
@@ -1223,7 +1224,8 @@ void entityclass::createentity(int xp, int yp, int t, int meta1, int meta2, int
         if (entities[i].invis
         && entities[i].size == -1
         && entities[i].type == -1
-        && entities[i].rule == -1)
+        && entities[i].rule == -1
+        && !entities[i].isplatform)
         {
             reuse = true;
             entptr = &entities[i];