Fix world going black and white in certain locations on apple gpu (#2558)

Author: Rye

indra/newview/app_settings/shaders/class2/deferred/alphaF.glsl

@@ -150,7 +150,7 @@ vec3 calcPointLightOrSpotLight(vec3 light_col, vec3 diffuse, vec3 v, vec3 n, vec
         float amb_da = 0.0;//ambiance;
         if (da > 0)
         {
-            lit = max(da * dist_atten,0.0);
+            lit = clamp(da * dist_atten, 0.0, 1.0);
             col = lit * light_col * diffuse;
             amb_da += (da*0.5+0.5) * ambiance;
         }

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

@@ -140,7 +140,7 @@ vec3 calcPointLightOrSpotLight(vec3 light_col, vec3 npos, vec3 diffuse, vec4 spe
         float amb_da = ambiance;
         if (da >= 0)
         {
-            lit = max(da * dist_atten, 0.0);
+            lit = clamp(da * dist_atten, 0.0, 1.0);
             col = lit * light_col * diffuse;
             amb_da += (da*0.5 + 0.5) * ambiance;
         }