Optimize post process rendering for gltf material preview

[RyeMutt]

Description

Optimizes post process rendering of GLTF material preview to remove unnecessary effects.

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Additional Notes

[cosmic-linden]

We could probably do without DoF and FXAA, but iirc exposure did help in some cases to improve the visibility of the material.

What is the motivation for this change? Is this for code cleanup? On the performance front, LLGLTFPreviewTexture::render should ideally not be rendering every frame, only when needed.

[RyeMutt]

Motivation was code cleanup to reduce the complexity of the material preview render. I can revert the exposure changes but ideally would like to eliminate DoF and FXAA as they're both relatively heavy post process passes that have little benefit here.

[cosmic-linden]

@RyeMutt That sounds reasonable.

[RyeMutt]

I tweaked it to not remove the exposure pass but did eliminate the screen space reflection copy as that also has no benefit here.

[cosmic-linden]

Sounds good to me.