SL-17274: PBR: Fix EMS being zero when using GGX approximation

indra/newview/app_settings/shaders/class3/deferred/softenLightF.glsl

@@ -23,6 +23,7 @@
  * $/LicenseInfo$
  */
 
+#define PBR_GGX_APPROX            1
 #define DEBUG_PBR_PACKORM0        0 // Rough=0, Metal=0
 #define DEBUG_PBR_PACKORM1        0 // Rough=1, Metal=1
 #define DEBUG_PBR_TANGENT1        1 // Tangent = 1,0,0
@@ -141,7 +142,9 @@ vec2 getGGX( vec2 brdfPoint )
 {
     // TODO: use GGXLUT
     // texture2D(GGXLUT, brdfPoint).rg;
+#if PBR_GGX_APPROX
     return getGGXApprox( brdfPoint);
+#endif
 }
 
 vec3 calcBaseReflect0(float ior)
@@ -274,6 +277,9 @@ void main()
         // Reference: getIBLRadianceLambertian
         vec3  FssEssLambert = specWeight * kSpec * vScaleBias.x + vScaleBias.y; // NOTE: Very similar to FssEssRadiance but with extra specWeight term
         float Ems           = (1.0 - vScaleBias.x + vScaleBias.y);
+#if PBR_GGX_APPROX
+              Ems           = alphaRough; // With GGX approximation Ems = 0 so use substitute
+#endif
         vec3  avg           = specWeight * (reflect0 + (1.0 - reflect0) / 21.0);
         vec3  AvgEms        = avg * Ems;
         vec3  FmsEms        = AvgEms * FssEssLambert / (1.0 - AvgEms);