Add rasterisation option

[william-hutchison]

Adds the argument rasterise_points to the function gate. When rasterisation is used, points are rasterised to an image before interactive gating is launched, improving performance for large datasets.

@stemangiola let me know if you would like to see any changes. If you approve and can push to CRAN, I will push teh matching update in tidySpatialExperiment at william-hutchison/tidySpatialExperiment#42 to the devel branch.