benchmarks: fix output formatting
benchmarks: fix output formatting

snprintf is a better tool for this task, anyway, but here's what was
wrong with the stream formatting:

s << name << ":" << std::setw(45 - (int)s.tellp())

cannot be used to align columns, tellp() may return any of:
    0
    name.length()
    name.length() + strlen(":")

because the compiler is allowed to reorder the evaluation of
sub-expressions as it likes -- it may evaluate s.tellp() before
evaluating (s << name) or in-between (s << name) and (s << ":")

try with gcc-4.8

zerebubuth/mapnik,naturalatlas/mapnik,mapnik/mapnik,CartoDB/mapnik,zerebubuth/mapnik,naturalatlas/mapnik,mapycz/mapnik,naturalatlas/mapnik,lightmare/mapnik,mapnik/mapnik,tomhughes/mapnik,pnorman/mapnik,lightmare/mapnik,lightmare/mapnik,mapnik/mapnik,tomhughes/mapnik,CartoDB/mapnik,pnorman/mapnik,mapnik/mapnik,tomhughes/mapnik,zerebubuth/mapnik,mapycz/mapnik,pnorman/mapnik,CartoDB/mapnik,lightmare/mapnik,tomhughes/mapnik,mapycz/mapnik,naturalatlas/mapnik,pnorman/mapnik