secondlife/viewer#3584 Fix avatar body mesh and system clothing conflicting with water and alpha due to missing discard of invisible pixels (FS:TJ cherry-pick to fix FIRE-36310)

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

@@ -51,9 +51,7 @@ in vec3 vary_norm;
 in vec4 vertex_color; //vertex color should be treated as sRGB
 #endif
 
-#ifdef HAS_ALPHA_MASK
 uniform float minimum_alpha;
-#endif
 
 uniform mat4 proj_mat;
 uniform mat4 inv_proj;
@@ -225,6 +223,13 @@ void main()
 
     float final_alpha = diffuse_linear.a;
 
+#ifdef IS_AVATAR_SKIN
+    if(final_alpha < minimum_alpha)
+    {
+        discard;
+    }
+#endif
+
 #ifdef USE_VERTEX_COLOR
     final_alpha *= vertex_color.a;