Fix grass block sides appearing white with custom resource packs (fixes #203, #373)

[anton-sjoberg]

Summary of Changes

Fixes #203
Fixes #373

This PR resolves issues where grass block side faces render white or display white/light-colored bands when using custom resource packs (e.g., Whimscape, Faithful) or at distant LOD mip levels.

Problem & Root Cause

Previously, quads.frag attempted to detect tinted grass overlay pixels using a heuristic grayscale test:
glsl if (abs(tintTest.r - tintTest.g) < 0.02f && abs(tintTest.g - tintTest.b) < 0.02f) { doTint = true; }
This heuristic suffered from multiple failure modes:

1. Custom Resource Packs: Many packs (such as Whimscape, Faithful, or packs with subtle saturation/pebbles in dirt) do not use pure grayscale values for the grass overlay or dirt background, causing the heuristic to fail and leaving grass sides un-tinted (white).
2. Mipmapping & Downsampling: Downsampling blends tinted overlay pixels with dirt pixels, producing non-gray intermediate RGB values and resulting in visible white bands along grass edges at LOD distances.
3. Shader Performance: An extra texture fetch ( extureLod(blockModelAtlas, texturePos, 0)) was performed per fragment in quads.frag.

Solution

1. Ground-Truth Tint Masking:
   • In ModelFactory.java, the exact grass overlay mask is captured during texture baking (partialTintMask).
   • For partial-tint faces, the alpha channel of the baked texture encodes the exact tint coverage (alpha = 255 for tinted grass fringe, alpha = 0 for untinted dirt).
   • Marked
     eedsAlphaDiscard = false for partial-tint faces so dirt pixels are never discarded.
2. Mipmap Generation (MipGen.java):
   • During downsampling, the alpha channel smoothly transitions to represent fractional tint coverage.
   • solidify() is bypassed for partial-tint faces to ensure dirt RGB colors are preserved across all mip levels without color bleeding.
3. Shader Modernization (quads.frag):
   • In both standard and Iris-patched shader paths, partial tinting now performs a smooth mix using the alpha channel:
     glsl colour.rgb *= mix(vec3(1.0), tintRGB, colour.a);
   • Guarded alpha discard so partial-tint quads are not prematurely discarded ( intingState() != 1u).
   • Removed the secondary texture fetch, reducing shader texture bandwidth.