Do not render highway areas in between the road layers

[matthijsmelissen]

We current render highway areas in between the road layers: highway areas are rendered above the casing of roads, and below the fill of roads. This is visually very nice, as it allows us to connect the casing of areas nicely to the casing of roads.

However, it also causes technical difficulties. Rendering highway areas in between the road layers adds a lot of complexity. It makes other technical issues much harder to fix, such as #2046. In addition, it leads to cartographic problems, such as #688 and #529.

Rendering highway areas below the roads would make the style technically much simpler. In particular, it would allow us to merge the roads-fill and roads-casing layers. Also, it would be a first step towards merging all road layers in the project.mml file.

By the way, as far as I know we are the only style (with the exception of styles derived from this one) that takes this approach. Other styles either render highway areas below roads like I propose (such as the humanitarian style), or do not render highway areas at all (like the Wikimedia style).

I therefore think we should start render highway areas below the roads, like we render landuse.

CC @pnorman for his opinion.

[jeisenbe]

Also related to #529

[pnorman]

it would be a first step towards merging all road layers in the project.mml file.

With what I was looking at for merging the layers was to have the areas as part of the road layer. Keeping areas as their own layer before other road layers would be much simpler.

[matthijsmelissen]

I'm currently working on this.