Show island and islet labels earlier

[kocio-pl]

Resolves #1448.

Island names are currently rendered on z7+, but they should be rendered on z4+ (as early as possible, but not compete with country labels on z3) and islets also 3 zoom levels earlier.

@rrzefox: Could you test this patch on your server?

Nunavut (Canada) example

z4
Before

After

[sommerluk]

Looks good to me. That includes the change for islet. Islet names only showed up only on high zoom levels, because of the 1-km²-threshold in the wiki documentation, but the chosen zoom level (14) did only work well on low latitude. For high latitude like http://www.openstreetmap.org/way/27923209#map=13/83.6622/-30.6165 clearly zoom level 11 is a better threshold.

[kocio-pl]

Rendering the planet with just the island names. This should help us find all the missing tagging - for example Spitsbergen should be fixed now (the planet data are older), but Sumatra exist only as an islet in Dubai and we lack New Guinea and Greenland for sure.

The list to check:
https://en.wikipedia.org/wiki/List_of_islands_by_area#Islands

z4

z5

z6

[rrzefox]

@rrzefox: Could you test this patch on your server?

It's been deployed. Comparison as usual on http://bl.ocks.org/math1985/raw/af7a602c222dbf1ff1a2c0d84ed755b7/#5.00/72.344/-97.588
Z0-9 rerendered, 10-14 still pending. all relevant zoom levels should have been rerendered by now (2017-10-20)