Improve plotting code for SOS (spline-on-the-sphere) smooths

[gavinsimpson]

See r-spatial/sf#2348 for a function that produces a nice sphere on a proper grid. Need to combine the code in this thread with existing code. The main issue is when we have data near the edges of the lat/long scale.

Currently I just create data that is evenly spread over lat/long. But when near the edges and we combine this with the tiles used to plot, where the grid points define the tile centres, the edges of the tiles can go beyond > 90 Latitude or > 180 longitude (in absolute value). So I need to be much more careful when creating these grids for this kind of smoother as I do want to use coord_sf for plotting and that requires I have proper geometry,