Add custom uniform block (ChunkFix) passthrough API (#2974)

[Wiz1991]

Summary

Implements the custom-UBO (ChunkFix) passthrough requested in #2974, so a mod can have its own std140 uniform block forwarded to the program Iris substitutes for its terrain pipeline when a shader pack is active.

Status: draft — compiles cleanly (:fabric:build) but is not yet verified in-game. Opening for design review and to give the assignee something concrete to build on. Details and open questions below.

The problem (per #2974)

When a shader pack is active and a mod assigns a terrain pipeline via IrisApi.assignPipeline(...), Iris substitutes its own ExtendedShader for that pipeline. That substituted program declares only Iris's own uniform blocks (ExtendedShader bind-group layouts), so a custom block the mod bound to the render pass via RenderPass.setUniform("ChunkFix", buffer) is silently discarded — nothing in Iris reads unknown UBO names off the pass. This is what breaks MaLiLib/Litematica's ChunkFix (texture-atlas size) block under shaders.

Approach

New API (minor revision bumped 3 → 4):

void registerCustomUniformBlock(RenderPipeline pipeline, String blockName, String glslDeclaration);

The mod passes the block name and its exact std140 GLSL declaration (Iris can't derive the layout, and the mod can't edit Iris's generated program, so both pieces cross the API). Then:

1. Registry (IrisPipelines) — resolves the pipeline to its ShaderKey at registration and stores the block(s) under that key.
2. Declaration injection (ShaderCreator) — injects the block's GLSL after the #version line of the substituted program's source, so the block exists in the linked program.
3. Optional per-draw bind (ExtendedShader + MixinGlCommandEncoder) — after Iris sets up the substituted program, for each registered block present on this draw's pass, binds the mod's GpuBufferSlice (GlBuffer.handle() + glBindBufferRange) to a binding point clear of Iris's own blocks.

Why this is safe for shared programs

A mod's terrain pipeline resolves to the same ShaderKey (e.g. TERRAIN_SOLID) that vanilla chunk terrain uses, so the substituted program is shared. The bind is therefore made optional: it only fires for draws whose GlRenderPass.uniforms actually contains the block. Vanilla terrain draws never setUniform("ChunkFix", …), so they are provably untouched — the block is declared-but-unbound-and-unused for them.

Files

• api/v0/IrisApi + apiimpl/IrisApiV0Impl — new method, revision → 4
• pipeline/IrisPipelines — registry keyed by resolved ShaderKey
• pipeline/programs/ShaderCreator — inject declaration after #version
• pipeline/programs/ExtendedShader + pipeline/programs/IrisProgram + mixin/MixinGlCommandEncoder — optional per-draw bind
• gl/IrisRenderSystem — bindBufferRange helper
• iris.accesswidener — widen GlRenderPass.uniforms (mirrors existing .samplers)

Open questions for review

• Binding-point selection is a fixed base (12) beyond Iris's own blocks rather than dynamically queried — is there a canonical free range to use?
• Declaration injection is a string insert after #version; a transformer-level injection may be preferable. An unused injected block could in principle be optimized out (then the bind is a safe no-op).
• Consumption semantics: this makes the data available to the substituted (shader-pack) terrain program, but that program won't call MaLiLib's sampleNearest(TextureSize) — so this is the plumbing half; the visual result still depends on the mod side. Paired with a MaLiLib change that registers the block; happy to iterate.

Refs #2974.

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