Workaround for mipmap issues on Windows.

I've been getting artifact issues when using OpenGL with Nvidia on
Windows; the result is that no mipmaps are autogenerated, resulting in
garbled textures. This ought to fix it.
Only center main window if config shown first.

If the configuration window is shown first we need to tell SDL to center
the window, else it will be off the side. If not we should rely on the
WM to handle positioning.