Actually use the plane distances we just computed

We don't get any of the speedup if we don't do this.

We also forget about any objects nearer the camera than the previous value except the groundcover we're just about to deal with.

Fixes https://gitlab.com/OpenMW/openmw/-/issues/7844

apps/openmw/mwrender/groundcover.cpp

@@ -95,6 +95,8 @@ namespace MWRender
                     {
                         // Other objects are likely cheaper and should let us skip all but a few groundcover instances
                         cullVisitor.computeNearPlane();
+                        computedZNear = cullVisitor.getCalculatedNearPlane();
+                        computedZFar = cullVisitor.getCalculatedFarPlane();
 
                         if (dNear < computedZNear)
                         {