Add alphaDither property for independent dither strength

[mvaligursky]

Add a new alphaDither property on StandardMaterial so alpha blending and opacity dithering can be active at the same time with independent strengths. Resolves #8764.

Previously, when opacityDither was enabled the dither threshold was driven by the same opacity scalar as alpha blending, so a material was effectively either alpha-blended or dithered — never both at meaningful strengths. The new alphaDither property feeds the dither path with its own alpha, decoupled from blend.

Changes:

• New alphaDither property on StandardMaterial (range [0, 1])
• New shader uniform material_alphaDitherScale (JS computes alphaDither / opacity and the GLSL/WGSL dither call site multiplies it into dAlpha)
• BC preserved via a getter fallback: a material that has never had alphaDither assigned reads back as opacity, so the dither pass uses the same alpha as before (scale = 1, bit-identical output)
• copy() patched to preserve the implicit-fallback state across material.clone()

API Changes:

• Added: StandardMaterial.alphaDither — number in [0, 1]. Set to 1.0 to disable dither without recompiling the shader, 0.0 to fully discard via dither. Reads back as opacity until explicitly assigned.

Examples:

• examples/graphics/dithered-transparency: rebuilt as a side-by-side comparison — left table never assigns alphaDither (legacy "opacity drives dither" behavior), right table is driven by an Alpha Dither slider (new independent control). Demonstrates both BC and the new feature in one view.

[Copilot]

Pull request overview

This PR introduces an alphaDither property on StandardMaterial to decouple opacity dithering strength from alpha blending (opacity), enabling both techniques to be used simultaneously with independent intensities (resolving #8764).

Changes:

• Added StandardMaterial.alphaDither (with implicit fallback to opacity until explicitly assigned) and clone/copy support for the implicit state.
• Added a new shader uniform (material_alphaDitherScale) and applied it at the dither call site in both GLSL and WGSL standard fragment frontends.
• Updated the dithered-transparency example to demonstrate legacy (implicit) vs new (explicit) alphaDither behavior side-by-side, including a new UI slider.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/scene/shader-lib/wgsl/chunks/standard/frag/stdFrontEnd.js	Multiplies dither alpha by uniform.material_alphaDitherScale in WGSL standard frontend.
src/scene/shader-lib/wgsl/chunks/standard/frag/opacity.js	Declares WGSL uniform material_alphaDitherScale.
src/scene/shader-lib/glsl/chunks/standard/frag/stdFrontEnd.js	Multiplies dither alpha by material_alphaDitherScale in GLSL standard frontend.
src/scene/shader-lib/glsl/chunks/standard/frag/opacity.js	Declares GLSL uniform material_alphaDitherScale.
src/scene/materials/standard-material.js	Adds alphaDither property (implicit fallback), copies implicit state during copy(), and sets material_alphaDitherScale parameter when dithering is enabled.
examples/src/examples/graphics/dithered-transparency.example.mjs	Reworks the example into left/right tables to demonstrate implicit vs explicit alphaDither behavior.
examples/src/examples/graphics/dithered-transparency.controls.mjs	Adds an “Alpha Dither” slider bound to data.alphaDither.

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