Fix SPTerrainLighting.frag missing SpecularMap support (#2654)

* Initial plan

* Fix SPTerrainLighting to sample SpecularMap like MultiPass shader

Co-authored-by: riccardobl <4943530+riccardobl@users.noreply.github.com>
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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: riccardobl <4943530+riccardobl@users.noreply.github.com>

Author: Copilot

jme3-terrain/src/main/resources/Common/MatDefs/Terrain/SPTerrainLighting.frag

@@ -3,6 +3,9 @@
 #import "Common/ShaderLib/Lighting.glsllib"
 
 uniform float m_Shininess;
+#ifdef SPECULARMAP
+  uniform sampler2D m_SpecularMap;
+#endif
 
 varying vec4 AmbientSum;
 varying vec4 DiffuseSum;
@@ -619,6 +622,20 @@ void main(){
     #endif
 
 
+    //-----------------------
+    // read shininess or specularColor from specularMap
+    //-----------------------
+    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; //assumes that specularMap is a gray-scale reflectivity/shininess map
+      #else
+        specularColor = specularMapColor;
+      #endif
+    #endif
+
     //-----------------------
     // lighting calculations
     //-----------------------
@@ -655,9 +672,9 @@ void main(){
             vec3 viewDir = normalize(-vPos.xyz);
         #endif
 
-        vec2 light = computeLighting(normal, viewDir, lightDir.xyz, lightDir.w  * spotFallOff, m_Shininess);       
+        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 * vec3(light.y);
+                            SpecularSum.rgb * specularColor.rgb * vec3(light.y);
     }
 
 }

jme3-terrain/src/main/resources/Common/MatDefs/Terrain/TerrainLighting.j3md

@@ -238,6 +238,7 @@ MaterialDef Terrain Lighting {
             DIFFUSEMAP_11_SCALE : DiffuseMap_11_scale
 
             USE_ALPHA : useDiffuseAlpha
+            USE_SPECULARMAP_AS_SHININESS : useSpecularMapAsShininess
         }
     }