Always write opaque fragments instead of relying on blending being off for translucent RTT: Arena

Resolves #5391 (closed)

I'm not super happy about the shader change as it adds overhead to everything, but the FFP version is good. I don't think there's anything else that can be done to fix this.

In the original engine, the solution to this kind of problem is just to make everything opaque in the character preview RTT, so we've had an OpenMW-exclusive feature all along without knowing about it.