Do not let Direct3D override Software rendering

This could be done more compact with a nested ?: , but I think this makes a lot clearer what's going on

It's either 
* this 
* disable the software rendering option in Direct3D,
* add the Software renderer as a seperate backend

I would actually like option #3 the most (although it does mix quite different semantics, one requires restart, the other can't be enabled when a game is loaded), but this was the first thing that came to mind