Bug fix for disappearing mesh roads and rivers

I was having this issue with our mesh road getting culled when the camera was close to one end and very close to one side of the road. This is a fairly hard to reproduce bug but here is the fix.

In function _generateSlices() in the files messRoad.cpp and river.cpp change the following line:

if( i == 0 )
{
   box.minExtents = slice.p0;
   box.maxExtents = slice.p2;
   box.extend( slice.pb0 );
   box.extend( slice.pb2 );
}

to:

if( i == 0 )
{
   box.minExtents = slice.p0;
   box.maxExtents = slice.p0;
   box.extend( slice.p2);
   box.extend( slice.pb0 );
   box.extend( slice.pb2 );
}