Add area rendering for amenity=prison

[polarbearing]

Isolating undisputed style from combined proposal #771
This provides rendering for areas of amenity=prison, and closes

https://trac.openstreetmap.org/ticket/2512

Prison was apparently implemented in xml and did not make it to carto.

The design adopts the hatching style we have for military areas/barracks, just in a neutral grey and vertical, associating bars, as in the original implementation. Hatching being used, as @mkoniecz pointed out, for rather inaccessible areas. Or, in this case, difficult to leave ;-)

Icons is powers of two and allows global alignment.

[matthijsmelissen]

I tested this and checked the code, which looks good.

The only thing I'm concerned about is that it adds yet another type of symbology to the map that might not be immediately clear to the user, for something that does not occur very often. A symbol combined with a neutral colour fill with a dashed outline might be clearer. Anyway that's for @gravitystorm to decide.

[polarbearing]

Relying too much on the outline might be a problem since these facilities would typically come with a wall or fence tagged on their border. Also, from Z17 the symbol amenity_prison.p.20.png is still being rendered in the middle, with a name that should be unambigious, see example here:

[vincentdephily]

FWIW I like the dashed rendering, which we already use for millitary. Since we render so many things, I don't think we can afford solid fill for everything.

[matkoniecz]

Can you rebase this pull request to a single commit?

[polarbearing]

rebased

[matkoniecz]

I like results, I tested it on prisons in my city and it works well. One more request - can you rebase with amending commit description? Now it is rather confusing as police landuse is not affected by this PR.

After this change I would recommend merging.

[matkoniecz]

@math1985

The only thing I'm concerned about is that it adds yet another type of symbology to the map that might not be immediately clear to the user, for something that does not occur very often. A symbol combined with a neutral colour fill with a dashed outline might be clearer.

Symbol is currently also appearing. I am not considering it as a "yet another type of symbology", rather as a variant of symbology used for landuse=military. I also encountered this kind of symbol used as "area with highly restricted access" on many other maps.

[polarbearing]

amended message

[matkoniecz]

@gravitystorm Are you opposed to hatching also in this case? This one is not so eye catching, and in my opinion style fits feature.

#1197 (comment)

[nebulon42]

The pattern here introduced a grey semi-transparent background. Other backgrounds of patterns were removed recently (see #1114). Could the transparency interact with other landuse colours? I think this should be changed to use polygon-fill.

edit: Ok, I saw that the transparency is required.