Read layerid and viewport from the FS payload correctly for all platforms

The hack we had for the fast linking mesh shader case was not considering some cases, and it was utterly broken for LNL. Since we no longer need to worry about the out of range case for layers, we can always read from the FS payload. Using the existing function for layerid, and adding a similar one for viewport index takes care of it for all platforms.

This doesn't yet deal with whatever is going on before we get to this point that makes layerid sometimes be an intrinsic and sometimes a varying, but it does fix things that need fixing right now.