I've done the discussed changes, here is new patch series.

- frei0r_multivalue.h was dropped and plugins fixed to use F0R_DOUBLE_PARAMS
- some info texts were updated
- building plugins was made Cairo_FOUND conditional
- rotation anchor point was added to "cairoaffineblend"

I dropped "cairoshape" for now because with F0R_DOUBLE_PARAMS it had
15 params i decided that it needs to be simplified.

To build these you need Cairo development files

sudo apt-get install libcairo2-dev

on Ubuntu.