Add Triplanar texture coordinate type

[jakeblakeley]

Summary

Adds a Triplanar texture coordinate type for both layers and masks. It projects the layer/mask source along the three object axes and blends the three samples by the surface normal, so it works with any source — images and procedural textures alike — in both Eevee and Cycles, back to Blender 2.80.

Controls (layers and masks)

• Projection Blend — transition softness between the projection axes.
• Expand — grows the coverage of the shown sides to counteract the fade that appears when some sides are hidden.
• Show +X / −X / +Y / −Y / +Z / −Z — per-side visibility, selected by the sign of the surface normal so each side of an axis can be toggled independently.

How it works

When a layer or mask uses the Triplanar coordinate, its source group is wrapped in a per-entity node tree that instances the actual source three times with swizzled object-space coordinates. This is a little expensive on weaker hardware so we may want to warn folks. Color always blends all three projections (so there is no stretching on faces that tilt away from a single axis), while the Show/Expand controls only fade coverage through the alpha weights. get_source_tree / get_mask_tree transparently unwrap the wrapper, so the rest of the addon (modifiers, linear nodes, baking, source editing) is unaffected.

It composes with the existing machinery: smooth bump reuses the same wrapper for its neighbor sources, and layer/mask duplication copies the inner source tree so instances are not shared. You can technically get this effect with the fake lighting approach, but its quite a long setup. This also exists in similar software like Substance Painter.

All the weight math is built from scalar Math, Separate/Combine XYZ and Mix nodes, so it evaluates identically in both engines with no version-specific fallbacks. The triplanar utility node groups are generated in lib.py following the existing programmatic node-group convention (like get_smooth_mix_node), so no changes to the binary lib*.blend files were needed.

I also included an expand slider since this is useful to get more of a certain axis, but it might be a bit much as you can get the same effect in other ways.

Commits

1. Bugfix — grouped source tree handling in check_layer_source and replace_layer_type. Both are pre-existing issues affecting any layer that uses a source group (e.g. with smooth bump), independent of this feature; split out for separate review.
2. Feature — the Triplanar coordinate type.

Testing

Verified on Blender 5.2: node-graph validity, coordinate switching round-trips without leaking wrapper trees, procedural and image layers/masks, smooth-bump interaction, layer type changes, duplication, and save/reload. Rendered per-face parity checks in both Eevee and Cycles confirm the projection, per-side hiding, and expand behavior. Compatibility with 2.80 is by construction (only nodes and Vector Math ops available since 2.80 are used). Test on Nvidia and Mac GPUs.

You might need to set "shade smooth" on low poly meshes to see the effect properly. Here's screenshots of the effect working with textures, as a mask, and with procedural effects:

[jakeblakeley]

Superseded by ucupumar#409 — moved directly to the upstream repo.