Add shader defines for prepass to support alpha test and dithered opacity

[slimbuck]

Summary

The prepass shader pass was sharing the same empty code path as the pick pass, which meant it had no shader defines set. This caused materials with alpha testing or opacity dithering to render incorrectly during the prepass because the relevant shader code was never compiled in.

This PR separates the prepass from the pick pass and adds a dedicated preparePrepassPass() method to LitShader that sets:

• LIT_ALPHA_TEST — enables alpha-test discard in the prepass
• LIT_BLEND_TYPE — sets the blend type so the shader can handle transparency correctly
• STD_OPACITY_DITHER — enables opacity dithering (matching the shadow pass behavior via opacityShadowDither)

Technical Details

• Adds the ditherNames import from constants.js
• Splits the SHADER_PICK / SHADER_PREPASS combined branch in generateFragmentShader() so each pass gets its own preparation
• The new preparePrepassPass() follows the same pattern as prepareShadowPass()

[Copilot]

Pull request overview

This PR fixes incorrect opacity handling in the prepass by giving the prepass its own shader-define preparation path (instead of sharing the “do nothing” pick-pass path). This ensures the prepass compiles the same alpha-test / opacity-dither branches that the material expects.

Changes:

• Added a dedicated preparePrepassPass() in LitShader to set prepass-specific defines.
• Split the SHADER_PICK || SHADER_PREPASS branch so SHADER_PREPASS runs its own preparation.
• Imported ditherNames to support STD_OPACITY_DITHER define selection for the prepass.

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