Make low-zoom highways use OpenMapTiles defaults

[ZeLonewolf]

This PR does the following:

• Adds z4 trunk roads in OpenMapTiles to render, with the same style as motorway
• Remove the road layer opacity block, which was not used/necessary
• Simplify the fill color logic for trunk roads
• Remove toll selectors for zooms <9 where toll attributes aren't in the underlying tiles

Before:

After:

[adamfranco]

Not sure if it's appropriate for this issue or a new one, but now that low-zoom trunks in the Trans-Canada highway are being displayed at z4, the brightness of the red for trunk looks more impactful than the motorway color:

at z5 the extra width of motorways makes them closer in terms of visual impact

and by z6 motorways look much more impactful.

The reason I mention this is that maybe the opacity lines that were removed maybe need to stay, but have the opacity gradient changed to not completely eliminate roads.

[ZeLonewolf]

Right, the color is wrong in this PR (hence still draft), but it's showing what roads should be shown at z4.

[adamfranco]

This is looking quite good, with one little potential issue.

Right at z4, the trunks are super faint: https://preview.americanamap.org/pr/1271/#map=4/43.59/-92.5\

By z4.5 they are reasonably visible: https://preview.americanamap.org/pr/1271/#map=4.49/45.65/-91.07

Is that the behavior you are intending?

[ZeLonewolf]

Ugh, looks like I regressed something