Switch UI drawing from GL to Thin3D. This activates the D3D9 path as well.

For Thin3D see https://github.com/hrydgard/native/pull/230

Our D3D9 path is broken and unmaintained since the end of the X360 port, but this will let us wake it back to life. 

This makes our UI run on Direct3D and turns on the Direct3D backend in our GPU emulation.

Just set

     [Graphics]
     GPUBackend = 1

to try out the D3D9 path with this.
Added a check instead.