[server] Render tiles in groups of 4x4
[renderer] Work around broken write-func handling in Cairomm
[job] Fixed prerendering
[statistics] Write statistics to file in bulk.
[request_manager] Added timing for prerendering.
[statistic] Add slcining time to measurement.
[statistic] Fixed initialisation and output
[statistic] Don't crash on shutdown when no performence log was specified
[statistic] Add tool to create bar graphs from performence logs