Fix SPTerrainLighting.frag missing SpecularMap support

[Copilot]

SPTerrainLighting.frag hardcoded m_Shininess and ignored SpecularMap/useSpecularMapAsShininess entirely, causing terrain materials that rely on those parameters to silently lose specular response when the SinglePass technique is active.

Changes

• SPTerrainLighting.frag: Added m_SpecularMap uniform declaration under #ifdef SPECULARMAP. Before the per-light loop, now computes specularColor (default vec4(1.0)) and finalShininessValue (default m_Shininess) by sampling the specular map — using .r as shininess when USE_SPECULARMAP_AS_SHININESS is set, otherwise as a specular color multiplier. Loop updated to pass finalShininessValue to computeLighting and apply specularColor.rgb to the specular contribution.

• TerrainLighting.j3md: Added USE_SPECULARMAP_AS_SHININESS : useSpecularMapAsShininess to the SinglePass technique's Defines block — it was already wired in the MultiPass technique but absent here.

The SinglePass shader now mirrors the MultiPass (TerrainLighting.frag) specular logic exactly:

vec4 specularColor = vec4(1.0);
float finalShininessValue = m_Shininess;
#ifdef SPECULARMAP
  vec4 specularMapColor = texture2D(m_SpecularMap, texCoord);
  #ifdef USE_SPECULARMAP_AS_SHININESS
    finalShininessValue = specularMapColor.r;
  #else
    specularColor = specularMapColor;
  #endif
#endif
// ...
vec2 light = computeLighting(normal, viewDir, lightDir.xyz, lightDir.w * spotFallOff, finalShininessValue);
gl_FragColor.rgb += DiffuseSum.rgb * lightColor.rgb * diffuseColor.rgb * vec3(light.x) +
                    SpecularSum.rgb * specularColor.rgb * vec3(light.y);

Original prompt

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This section details on the original issue you should resolve

<issue_title>SPTerrainLighting does not sample SpecularMap</issue_title>
<issue_description>
Changing the default here makes Common/MatDefs/Terrain/TerrainLighting.j3md pick its LightMode SinglePass technique by default (jme3-terrain/src/main/resources/Common/MatDefs/Terrain/TerrainLighting.j3md:179-240). That path still does not implement terrain specular maps: SPTerrainLighting.frag never samples m_SpecularMap and always uses m_Shininess directly (jme3-terrain/src/main/resources/Common/MatDefs/Terrain/SPTerrainLighting.frag:5,649-651), whereas the current MultiPass shader applies both SpecularMap and useSpecularMapAsShininess (jme3-terrain/src/main/resources/Common/MatDefs/Terrain/TerrainLighting.frag:642-651). Any existing terrain material that relies on those parameters will therefore lose its specular response after this default flips, even though the app never opted into single-pass lighting.

Originally posted by @chatgpt-codex-connector[bot] in #2652 (comment)
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes SPTerrainLighting does not sample SpecularMap #2653

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[riccardobl]

I’ve never used the jME terrain system, so I’m not sure whether this PR is solid.
@yaRnMcDonuts, when you have some time, could you take a look? You might have better insight than I do.