Add support for SDF icons #77
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Missing tests, plus the calculation of the alpha value isn't quite right yet.
From the sdf_glyph_renderer crate. It actually does the maths properly, unlike my original implementation.
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See #58.
Mapbox and MapLibre both support SDF icons, sprites that store a signed distance field in their alpha channel. They're not well documented (Mapbox calls them re-colourable images and MapLibre doesn't really mention them) and they're pretty much abandoned as an idea (see the conversation in mapbox-gl-style-spec#444). But nevertheless both style specs support them and so Spreet should be able to generate SDF spritesheets.
Stadia Maps's
sdf_glyph_renderercrate supports signed distance field generation and so we can use that to do the heavy lifting. Although in theory a single spritesheet can mix-and-match SDF sprites with non-SDF sprites, in practice spritesheets tend to include one or the other but not both. And so for now I think it's fine for support to be all-or-nothing: a spritesheet can have all SDF icons, or no SDF icons. If there's demand in the future, we can support mixing.