[openstreetmap/openstreetmap-website] Status of picture/icon methods in lib/gpx.rb (#281)

[mmd]

Rendering a GPX trace icon isn't too difficult using gd2-ffij:

```ruby
    require 'gd2-ffij'

[...]

    def icon(min_lat, min_lon, max_lat, max_lon)
      width = 50
      height = 50
      proj = OSM::Mercator.new(min_lat, min_lon, max_lat, max_lon, width, height)

      image = GD2::Image::IndexedColor.new(width, height)

      black = image.palette.allocate(GD2::Color[0, 0, 0])
      white = image.palette.allocate(GD2::Color[255, 255, 255])

      image.draw do |pen|
        pen.color = white
        pen.rectangle(0, 0, width, height, true)
      end

      image.draw do |pen|
        pen.color = black
        pen.anti_aliasing = true
        pen.dont_blend = false

        oldpx = 0.0
        oldpy = 0.0

        first = true

        points do |p|
          px = proj.x(p.longitude)
          py = proj.y(p.latitude)

          pen.line(px, py, oldpx, oldpy) unless first

          first = false
          oldpy = py
          oldpx = px
        end

      end
  
      image.gif
    end
  end
```

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