way_area vs way_pixels

[matkoniecz]

way_area is not an intuitive unit, it is quite easy to convert it in SQL to in my opinion much better way_pixels[1]. way_pixels also converts nicely from zoom level to zoom level allowing less complicated conditional structures. I propose to replace all usage of way_area in .mss files by way_pixels.

It would affect only amenity-points (10 occurrences - for leisure=playground and displaying names of water bodies).

Related to #710

[1]way_area/(!pixel_width!*!pixel_height!) AS way_pixels

[pnorman]

duplicate of #703 (see #703 (comment))