Coastlines missing on cloud top height spatial plots

### Describe the bug

In https://github.com/MetOffice/CSET/pull/2045#issuecomment-4298187254 coastlines were removed when data is masked, with the intent of not having them when we also have a land-sea mask in the data. However, there are other fields that use masks that don't follow coastlines, such as cloud top height:

Having the coastlines missing makes it hard to locate the clouds. This effect is likely to extend to other variables that use masking but don't follow the land/sea.

### How to reproduce

Steps to reproduce the behaviour:

1. Create a spatial plot of `cloud_top_altitude`
2. Observe that it doesn't have coastlines.

### Expected behaviour

Fields that don't mask along the coast should plot the coastlines.

Start from the spatial plotting path used to render cloud_top_altitude and trace how masked fields are handled after the change described in PR 2045. Reproduce the plot, then verify that fields whose masks do not follow the coast retain coastlines while land-sea-masked fields preserve their intended behavior.

Written by the indexing model from the issue text.