Prefer SDL+OpenGL over DirectX

This changes arx to use the SDL windowing and input backends as well
as OpenGL for rendering, unless the user has explicitly  selected the
DirectX variants.

It looks like we will likely remove the D3D renderer at some point,
so this will give the GL renderer some more exposure and testers before
the option goes away.
Also, there are D3D-only issues (bug #422) but I'm not aware of any
OpenGL-only issues.