Put groundcover alphafunc where shader visitor can see it

I'd already made this change so don't know why it disappeared instead of
being included in b8ee32e3

apps/openmw/mwrender/groundcover.cpp

@@ -1,5 +1,6 @@
 #include "groundcover.hpp"
 
+#include <osg/AlphaFunc>
 #include <osg/Geometry>
 #include <osg/VertexAttribDivisor>
 
@@ -265,6 +266,9 @@ namespace MWRender
             group->addChild(node);
         }
 
+        // Force a unified alpha handling instead of data from meshes
+        osg::ref_ptr<osg::AlphaFunc> alpha = new osg::AlphaFunc(osg::AlphaFunc::GEQUAL, 128.f / 255.f);
+        group->getOrCreateStateSet()->setAttributeAndModes(alpha.get(), osg::StateAttribute::ON);
         group->getBound();
         group->setNodeMask(Mask_Groundcover);
         mSceneManager->recreateShaders(group, "groundcover", false, true);

apps/openmw/mwrender/renderingmanager.cpp

@@ -3,7 +3,6 @@
 #include <limits>
 #include <cstdlib>
 
-#include <osg/AlphaFunc>
 #include <osg/Light>
 #include <osg/LightModel>
 #include <osg/Fog>
@@ -312,10 +311,6 @@ namespace MWRender
             groundcoverRoot->setName("Groundcover Root");
             sceneRoot->addChild(groundcoverRoot);
 
-            // Force a unified alpha handling instead of data from meshes
-            osg::ref_ptr<osg::AlphaFunc> alpha = new osg::AlphaFunc(osg::AlphaFunc::GEQUAL, 128.f/255.f);
-            groundcoverRoot->getOrCreateStateSet()->setAttributeAndModes(alpha.get(), osg::StateAttribute::ON);
-
             mGroundcoverUpdater = new GroundcoverUpdater;
             groundcoverRoot->addUpdateCallback(mGroundcoverUpdater);