DRTVWR-4976 Misc shader cleanup.

Removed some potential div-by-0 NaNs and a mangled clamp.

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

@@ -99,11 +99,11 @@ void main()
     altitude_blend_factor = clamp((rel_pos.y + 512.0) / max_y, 0.0, 1.0);
 
     // Set altitude
-    if (rel_pos.y > 0.)
+    if (rel_pos.y > 0)
     {
         rel_pos *= (max_y / rel_pos.y);
     }
-    else
+    if (rel_pos.y < 0)
     {
         altitude_blend_factor = 0;  // SL-11589 Fix clouds drooping below horizon
         rel_pos *= (-32000. / rel_pos.y);

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

@@ -71,12 +71,12 @@ void main()
     // Get relative position
     vec3 rel_pos = position.xyz - camPosLocal.xyz + vec3(0, 50, 0);
 
-    // Set altitude
-    if (rel_pos.y > 0.)
+    // Adj position vector to clamp altitude
+    if (rel_pos.y > 0)
     {
         rel_pos *= (max_y / rel_pos.y);
     }
-    else
+    if (rel_pos.y < 0)
     {
         rel_pos *= (-32000. / rel_pos.y);
     }

indra/newview/app_settings/shaders/class1/windlight/atmosphericsFuncs.glsl

@@ -49,7 +49,7 @@ void calcAtmosphericVars(vec3 inPositionEye, vec3 light_dir, float ambFactor, ou
     vec3 rel_pos = inPositionEye;
 
     //(TERRAIN) limit altitude
-    rel_pos.y = clamp(rel_pos.y, -max_y, max_y);
+    if (abs(rel_pos.y) > max_y) rel_pos *= (max_y / rel_pos.y);
 
     vec3  rel_pos_norm = normalize(rel_pos);
     float rel_pos_len  = length(rel_pos);

indra/newview/app_settings/shaders/class2/windlight/cloudsV.glsl

@@ -99,12 +99,12 @@ void main()
     // fade clouds beyond a certain point so the bottom of the sky dome doesn't look silly at high altitude
     altitude_blend_factor = clamp((rel_pos.y + 512.0) / max_y, 0.0, 1.0);
 
-    // Set altitude
+    // Adj position vector to clamp altitude
     if (rel_pos.y > 0.)
     {
         rel_pos *= (max_y / rel_pos.y);
     }
-    else
+    if (rel_pos.y < 0.)
     {
         rel_pos *= (-32000. / rel_pos.y);
     }

indra/newview/app_settings/shaders/class2/windlight/skyV.glsl

@@ -67,12 +67,12 @@ void main()
     // Get relative position
     vec3 rel_pos = position.xyz - camPosLocal.xyz + vec3(0, 50, 0);
 
-    // Set altitude
+    // Adj position vector to clamp altitude
     if (rel_pos.y > 0.)
     {
         rel_pos *= (max_y / rel_pos.y);
     }
-    else
+    if (rel_pos.y < 0.)
     {
         rel_pos *= (-32000. / rel_pos.y);
     }