Use the prepass normal texture in main pass when possible

[IceSentry]

Objective

• We support enabling a normal prepass, but the main pass never actually uses it and recomputes the normals in the main pass. This isn't ideal since it's doing redundant work.

Solution

• Use the normal texture from the prepass in the main pass

Notes

I used NORMAL_PREPASS_ENABLED as a shader_def because NORMAL_PREPASS is currently used to signify that it is running in the prepass while this shader_def need to indicate the prepass is done and the normal prepass was ran before. I'm not sure if there's a better way to name this.

[superdump]

I suppose maybe LOAD_PREPASS_NORMALS could be clearer?

[IceSentry]

Oh yeah, good call, I'll rename it to that

[JMS55]

Yay GBuffer :)