Investigate Halo around Icons

[ingalls]

Context

Icons often don't have a high degree of contrast with the base map, investigate adding a halo affect to the icons to increase visibility

This could be accomplished one of two possible ways

• Investigate support for a render-side halo effect in GL - I want to avoid another layer so if it is required I'm less keen on this approach
• Automatically add a halo affect on the backend during spritesheet generation

[bobgann]

This would help us significantly with accessibility. Also the text is too small - it can be zoomed, but if we can make the default a little bigger

[ingalls]

Maplibre supports this for icons in the SDF format.

Ref: https://maplibre.org/maplibre-style-spec/sprite/#optional-properties

Work would likely require

• Automatically detecting eligible PNGs for conversion to SDF - not all would be
• Converting to SDF and adding sdf: true to sprite json output.