[core] Rationalize style property classes

[jfirebaugh]

This is the first in a series of PRs that builds towards a full-fledged programmatic style API. This is a big chunk of work, but I'm trying to break it up step by step and not create a long-lived branch with a lot of changes.

Overall, my first major goal is to eliminate ClassProperties -- this is essentially an untyped map of every possible style property. Instead, we should work with classes like:

class FillPaintProperties {
public:
    bool antialias = true;
    float opacity = 1.0f;
    Color fill_color = {{ 0, 0, 0, 1 }};
    Color stroke_color = {{ 0, 0, 0, -1 }};
    std::array<float, 2> translate = {{ 0, 0 }};
    TranslateAnchorType translateAnchor = TranslateAnchorType::Map;
    Faded<std::string> image;

    inline bool isVisible() const {
        return opacity > 0 && (fill_color[3] > 0 || stroke_color[3] > 0);
    }
};

These classes already exist, they are just inconsistently named. This PR makes the naming both internally consistent, and consistent with terminology from the style specification: for each layer type, there is a ____PaintProperties class and a ____LayoutProperties class. For the time being, all of these classes live in style_properties.hpp.

In code that I touch, I'm also fixing style issues like & placement and variable naming conventions. This will be the case with the whole series of PRs.

👀 @kkaefer @brunoabinader

[kkaefer]

Can we rename this to StylePaintProperties?

[jfirebaugh]

It's going away in the next commit, so I held off.

[kkaefer]

Looks good!

[brunoabinader]

It is great that now layout properties are now exported to the public API. 👍 !