Pdf rendering: try to reduce memory consumption: don't load the fonts in memory, use temp files

[syjer]

see d814a3a .

Note, we must wait the next release of openhtmltopdf so that danfickle/openhtmltopdf#215 is included.

[syjer]

actually: I've committed a fix to only load on demand: df7179d .

But still, we need to measure the effective memory consumption of the font embedding process/pdf generation and find a way to reduce it: we will need to check pdfbox and openhtmltopdf.

I'll leave open this task.

Edit: this commit does not work when launched from the executable war... Will need a new release of openhtmltopdf.

[cbellone]

can be considered done