Make regional admin borders solid instead of dashed

[matthijsmelissen]

Due to the coastline paradox, dashed lines for borders don't work
well on low-zoom: strongly curved borders appear solid instead of
dashed. Also, solid dashes generally are too busy for the crowded
z5-7.

This PR makes regional boundaries (admin_level=4) solid instead of
dashed up to z9.

To prevent the borders from getting too strong, they are also made
thinner.

Before/after:

z4:

z5:

z6:

z7:

z8:

z9:

[kocio-pl]

Thanks, it's a really useful change!

Maybe this makes it more solid and violet+red is a bit more dominating on low zoom (see #1935), but low zoom changes are still ahead of us (#2688) and we can try with borders color change (like #2695) and at least it stops looking like artefacts (another kind of high frequency noise to remove - see also #2199):

Poland, z6
Before

After

On the other hand I'm not sure to which zoom level it should be used. You were testing in Africa, but in Europe we have more linear entities (like roads, railways and rivers), so maybe z7-z9 should be left untouched to be more distinct.

Poland, z9
Before

After

[kocio-pl]

This is great change for z<7, but z7+ should be left a it is to be more distinct from other linear features.

[pnorman]

The changes look good, but I haven't run them, and would want to look at higher zooms locally.

[matthijsmelissen]

I agree with @kocio-pl. I restricted the PR to zoom level 4-6. Merged.