Simplify logging trace when drawing at low zoom level

[M4rtinK]

We don't really have to draw all logging trace points when we are drawing a trace at low zl - we can basically skip points based on current zoomlevel (the GTK GUI & Tango GPS do that via modulo).

We could also do a check of pixel distance after switching zoom levels and filter based on that but that might be too complicated/overkill.

BTW, the modulo based algorithm can't be used for routes returned from automated routers - as they already contain only the necessary points it would lead to in most cases to very visible information loss. On the other hand logging traces contain points recorded in regular intervals - in this case just using every second/third/etc. point is much less apparent at the resulting polyline.