[lmi] PDF generation performance

[Vadim Zeitlin]

 Hello,

 While testing the pagination anomaly bug, I couldn't help noticing how
awfully long PDF generation took for the census used to reproduce the
problem. With the (optimized) MSVS build, it took ~40 seconds and while I
didn't measure it exactly with MinGW, it doesn't seem to be any faster.
And in the debug MSVS build it's ~200s which is nice because it allows me
to get a coffee while it's running, but not from any other point of view.

 I will, of course, profile the code to see if I can speed it up in its
current form, but it seems clear that to achieve any really important
gains, as we'd need to make this more bearable, we have to parallelize the
output files generation.

 Would you agree that doing this is important?
VZ