Remove 'Meshbuffer ran out of indices' limitation

This fixes the "FIXME: Meshbuffer ran out of indices" problem that occurs when a mapblock mesh becomes too complicated (for example if the mapblock is filled with nodes with drawtype allfaces/allfaces_optional - this has been reported to happen with moretrees - or when too many nodeboxes are used in one mapblock).

Solution: If the data passed to MeshCollector::append would not fit into the existing meshbuffers, allocate a new meshbuffer.