Consider zoom when culling billboards.

screenScale represents the current zoom factor, but may need some
revision because I'm not sure if what it's doing is technically correct.
It is used to multiply the culling distance for cells, as well as the
fading distance.

Engine/source/T3D/fx/groundCover.cpp

@@ -1550,8 +1550,10 @@ void GroundCover::prepRenderImage( SceneRenderState *state )
    {
       PROFILE_SCOPE( GroundCover_RenderBillboards );
 
+      F32 screenScale = state->getWorldToScreenScale().y / state->getViewport().extent.y;
+
       // Set the far distance for billboards.
-      mCuller.setFarDist( mRadius );   
+      mCuller.setFarDist( mRadius * screenScale );
 
       F32 cullScale = 1.0f;
       if ( state->isReflectPass() )
@@ -1560,7 +1562,7 @@ void GroundCover::prepRenderImage( SceneRenderState *state )
       // Setup our shader const data.
       // Must be done prior to submitting our render instance.
 
-      mShaderConstData.fadeInfo.set( mFadeRadius * cullScale, mRadius * cullScale );      
+      mShaderConstData.fadeInfo.set( mFadeRadius * cullScale * screenScale, mRadius * cullScale * screenScale );    
 
       const F32 simTime = Sim::getCurrentTime() * 0.001f;