Style-driven label field

[yhahn]

Per voice, confirm that llmr-native is not drawing text from a hardcoded name field but can utilize other fields.

[yhahn]

This is working but not for labeling contour labels (ele field) which is ... either a numeric string or not a string at all. Seeing these errors:

[WARNING] missing shaping for '3000'

Will need to dig.

[yhahn]

name_en for country labels.

[yhahn]

^^ eliminates errors but no contour labels showing yet. Not sure if it's a style def issue or in the renderer.

[yhahn]

@mikemorris is investigating the contour labels.

[mikemorris]

@yhahn Earlier suspicions about std::u32string format were unfounded, I have no clue why these aren't rendering. Elevation labels are getting to at least https://github.com/mapbox/llmr-native/blob/master/src/renderer/text_bucket.cpp#L61 so it looks like they're actually getting pushed onto the vertex buffer, not quite sure how to debug further.

[kkaefer]

This turned out to be a missing style in the stylesheet. The bucket and the layer was there, but there was no associated stylesheet.

I changed the renderer so that it uses defaultProperties when it can't find a style, instead of not rendering at all.