SL-18790 Fix for transparent objects casting shadows when they ought not.

indra/newview/app_settings/shaders/class1/deferred/shadowAlphaMaskF.glsl

@@ -48,6 +48,24 @@ void main()
         discard;
     }
 
+#if !defined(IS_FULLBRIGHT)
+    alpha *= vertex_color.a;
+#endif
+
+    if (alpha < 0.05) // treat as totally transparent
+    {
+        discard;
+    }
+
+    if (alpha < 0.88) // treat as semi-transparent
+    {
+        if (fract(0.5*floor(target_pos_x / post_pos.w )) < 0.25)
+        {
+            discard;
+        }
+    }
+
+
 	frag_color = vec4(1,1,1,1);
 	
 #if !defined(DEPTH_CLAMP)