[Patch] improve performance in Renderer

[GerdP]

I've noticed that MatchingReferrerFinder.visit(Way w) is using a sequential search
through all way nodes to find the index of the node. This index is only used when the
corresponding style rule uses it, in the default file elemstyles.mapcss no rule uses
it. The same seq. search is done for members of relations, and again, no default rule
uses this. I've created a quick hack which avoids the search when the only information
that is needed is whether the parent way (relation) matches.

I guess a cleaner solution would be to add a boolean field indexNeeded in the selector
which would be set when the rule contains the index clause (or the role clause).
I leave that to the experts.