add batch map conversion stage to find some unnecessary heightfields

In old versions of AC it was necessary to trick with heightfields to raise
a pickup entity. However, for quite some time now, pickups can be raised
by attribute.

This additional stage for convmap finds pickup entities where the
heightfield is most probably unnecessary. Fixing is manual work, though.

Updated those pickups on 16 maps.

Also fixed a playerstart on desert3

source/src/world.cpp

@@ -387,7 +387,7 @@ void newentity(int index, int x, int y, int z, const char *what, float v1f, floa
         case LIGHT: calclight(); break;
         case SOUND: audiomgr.preloadmapsound(e); break;
     }
-    unsavededits++;
+    if(index >= 0 || type != DUMMYENT) unsavededits++;      // no need to save dummies
 }
 
 void entset(char *what, float *a1, float *a2, float *a3, float *a4)