Fixing `nan_color` for CairoMakie when plotting heatmaps

Since you can provide your own masking function that NaN out data, you may want to change the color of the NaNs to something else. With the CairoMakie backend, you can't easily change it by using the `nan_color` keyword. See [https://github.com/MakieOrg/Makie.jl/issues/4524](https://github.com/MakieOrg/Makie.jl/issues/4524).

To fix this, we can do what is suggested in that issue and plot it like this: `surface!(ax, lons, lats, zeros(size(field)); color = field, shading = NoShading, nan_color = :red)`. Here, we are using the `color` keyword.

No file or test is named. Start by locating the heatmap or surface plotting entry point and its CairoMakie-specific path, then review the linked issue for context. Done means heatmaps rendered with CairoMakie honor the nan_color keyword when color is supplied as described.

Written by the indexing model from the issue text.