do not render icon for private playgrounds

[matkoniecz]

Maybe area also should not be rendered.

example: https://www.openstreetmap.org/way/290569371

related to #312, #635, #634

[pnorman]

opacity like with parking might be reasonable here. easier to do with SVG, since we don't need two files then.

[matthijsmelissen]

In Luxembourg, most school playgrounds are open to the public after school hours. Would these playgrounds count as public playgrounds?

[matkoniecz]

Probably yes - opening_hours tag would be more useful in this case.

[matkoniecz]

#1012 is better