[openrailwaymap] Suggestion for openrailwaymap.org rendering: Use maxspeed instead of layer

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For the speed theme, I suggest to render faster lines on top of slow ones. I guess that the speed theme is used by people who want to find which and where the fast lines are. Currently, this doesn't work well: E.g. the Lötschbergbasistunnel at 250 km/h is not visible in many zoom levels, since the slower Lötschberg-Bergstrecke is visible instead (in some zoom levels, the number "250" from the Lötschbergbasistunnel is visible, but the color is the 80 km/h turquoise of the Lötschberg-Bergstrecke. The problem is also widespread in Austria.

Philipp