Respect the presence of holes in calculated contours

[thomasp85]

The current implementation of stat_contour_filled() does not respect the presence of holes in the contour as it does not provide a subgroup column. This PR fixes this:

Before:

library(ggplot2)

volcano_long <- data.frame(
  x = as.vector(col(volcano)),
  y = as.vector(row(volcano)),
  z = as.vector(volcano)
)

ggplot(volcano_long, aes(x, y, z = z)) + 
  geom_contour_filled(aes(fill = stat(level)), bins = 8) + 
  scale_fill_brewer()

^{Created on 2020-01-08 by the reprex package (v0.3.0)}

After:

library(ggplot2)

volcano_long <- data.frame(
  x = as.vector(col(volcano)),
  y = as.vector(row(volcano)),
  z = as.vector(volcano)
)

ggplot(volcano_long, aes(x, y, z = z)) + 
  geom_contour_filled(aes(fill = stat(level)), bins = 8) + 
  scale_fill_brewer()

^{Created on 2020-01-08 by the reprex package (v0.3.0)}

This PR also makes sure that fill can be partly transparent without building up opacity since contained levels are cut out

[clauswilke]

lgtm