NME: Support core SFE mode

[alexchuber]

This PR adds everything needed to support the “happy path” for creating SFE-compatible shaders in the Node Material Editor. In other words, all the core pieces are in place to build a default SFE NodeMaterial, as long as everything is set up correctly.

That said, there aren’t any UI guardrails yet — it’s still possible to break things or go off track. Guardrails and validation will come in future PRs.

• Add ScreenUVBlock, which corresponds to the UV varying passed by postprocess.vertex, which is used by Smart Filters
• Reuse NodeMaterialModes.PostProcess logic for creating the effect in the preview canvas. This avoid adding any more SFE-specific logic to NodeMaterial proper.
• Add Export to SFE button

[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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