Change text-poly-low-zoom to use ST_PointOnSurface

[jeisenbe]

Related to #3201 - migration to server-side vector tiles

Changes proposed in this pull request:

• Change text-poly-low-zoom layer to use ST_PointOnSurface - this is necessary to use this style with vector tiles, and this makes the low zoom rendering consistent with the label placement at higher zoom levels (z10 and up
• Also remove unneeded text-placment: interior for the polygon features affected - this code is no longer needed, because the labels are placed on a point, not in a polygon, by Mapnik at the time of rendering.

Test rendering:

z8 Libya
https://www.openstreetmap.org/#map=8/27.330/17.216

z9 Wan am namus
https://www.openstreetmap.org/#map=9/24.8628/17.7742

[matthijsmelissen]

AND has higher precedence than OR In SQL, so we need a pair or brackers around all the OR statements.

[jeisenbe]

In addition to the pair here?

  AND (landuse IN....
...
  OR leisure IN ('nature_reserve'))

Where should the brackets be? It seems to work currently

[pnorman]

The brackets look good to me as-is.

[jeisenbe]

@matthijsmelissen - the current code works. I don't see where the brackets are missing. Could you advise?

[jeisenbe]

@matthijsmelissen there is a pair of brackets around all the OR statements:

WHERE way && !bbox!
AND (landuse IN ('forest', 'military', 'farmland')
  OR military IN ('danger_area'
...
  OR leisure IN ('nature_reserve'))
AND building IS NULL
...

[jeisenbe]

@matthijsmelissen could you respond to the comments above?

[jeisenbe]

@matthijsmelissen please respond to my answer to your comment above, it has been 3 months now.

[matthijsmelissen]

I might have mislooked, brackets look fine to me.

[jeisenbe]

Thank you for the help. If you have time, would you be able to approve this PR?