Fix to overlays issue 2648

Squashed commit of the following:

commit c5b72beed4f79675191507a31757b698b6e856d8
Author: vs <vs2009@mail.com>
Date:   Sun Nov 21 20:46:05 2021 +1000

    Terrain shaders: Fix terrain overlays not rendering correctly with overlays enabled (WS2 terrain shaders, not WS3). Fixes issue 2648: https://sourceforge.net/p/flightgear/codetickets/2648/ . The 1st pass for the overlays technique is a copy of the ALS ultra shaders pass, and it needed an update adding a new uniform.

Effects/terrain-default.eff

@@ -193,6 +193,11 @@
   <!-- put techniques at a "high" index to allow derived effects to
        insert their own techniques first. -->
 
+
+   <!-- 
+        ALS ultra settings technique. The 1st pass of terrain-overlay.eff is a copy of
+        this pass, and changes should be made there as well.
+   --> 
 <technique n="3">
     <predicate>
       <and>

Effects/terrain-overlay.eff

@@ -1,5 +1,13 @@
 <?xml version="1.0" encoding="utf-8"?>
 <PropertyList>
+<!-- 
+     WS30 terrain overlay effect. There are 2 passes over the terrain. 
+       The 1st pass is a normal ALS ultra shaders pass from ws30.eff. 
+         This pass should be kept up to date with ws30.eff.
+       The 2nd pass uses geometry shaders to add smaller scale volumetric detail with 
+         simplified lighting. 
+-->
+
     <name>Effects/terrain-overlay</name>
     <inherits-from>Effects/terrain-default</inherits-from>
 
@@ -664,6 +672,11 @@
         <type>sampler-2d</type>
         <value type="int">2</value>
       </uniform>
+          <uniform>
+            <name>swatch_size</name>
+            <type>int</type>
+            <value><use>xsize</use></value>
+          </uniform>
       <uniform>
         <name>colorMode</name>
         <type>int</type>