Draw the plot and facets

- Plotting in the `ggplot`, `layer` and `geom` objects
- Introduce new (possibly still evolving) API i.e
  `draw_group()` and `draw` for the geoms
- Update `geom_point` and use rgba array to avoid
  groupbys on alpha mappings.
- Fix missing `nrow` and `ncol` in the facets
- Draw facet labels