Allow empty rasters in `rasterplot`

This is useful for generating animations.
Improvements to `rasterplot`

- Made the rasters with and without `eventplot` look more similar.
- Gave users the option to choose whether `eventplot` is used.
  This is useful if you want to enforce consistency across machines,
  for example by forcing `rasterplot` to not use `eventplot` so that
  the plot looks the same no matter the Matplotlib version.
- Added an optional test/benchmark for `rasterplot` that just makes
  plots both with and without `eventplot` for easy assessment.