Replace deprecated alpha test in shader visitor

My suspicion is that this will marginally increase loading and culling time, but there's a small chance it'll decrease draw time. If we're especially lucky, the increased times won't be measurable and the decreased times will, so we might even see a net benefit.

That means I need performance information. The impact might be heavily vendor/driver-specific.

Outstanding issues:

• None known. :)

Resolves #4899 (closed)