Improve multiple MF2_LINKDRAW objects in Software

Previously, attaching multiple objects with MF2_LINKDRAW to one object would randomly sort them. Now, these objects will get sorted against each other properly.

The base game only ever uses MF2_LINKDRAW for MT_TAILSOVERLAY, and the behavior is intended to be unchanged for those kinds of single objects. This is just a simple fix to make the behavior more usable for mods, and to make Software act a little more like OpenGL.