Remove unused panels for subsetted geoms

For geoms using the subset argument, ensure no more panels than required
are used.

e.g.

    d = data.frame(
      id=as.factor(rep(c('A', 'B', 'C'), 10)),
      x=ceiling(seq(1, 30) / 3), y=runif(30)
    )
    ggplot(d, aes(x=x, y=y, group=id, color=id)) +
    geom_point(subset=.(id == 'A')) +
    geom_line(subset=.(id == 'C')) +
    facet_grid(id ~ .)
Turned off panel subsetting if facet has drop=FALSE