[Tilesathome] less inkscape runs

[spaetz]

Render improvements:

Somebody timed inkscape on a German forum and found that loading the
SVG takes most of the time, and exporting to png is rather
quick. Given that inkscape seems to render the whole SVG internally
anyway, even if we export only a small excerpt to png, I was
wondering if we should not make use of that fact, and simply export
the whole svg to one png in one render go. It could then be split up
into the tiles....

Of course that doesn't help with memory consumption in Boston
downtown, but it would save us:
1 render go at z13
3 at z14
7 at z15
15 at z16
31 at z17.


So we'd need only 6 inkscape calls, rather than 63 now (if I haven't
gotten that wrong). This could be quite a timesaver. Worthwhile
exploring? It could also be done differently, depending on the tileset complexity
measure we get from the server.

spaetz