[Cin] Blur plugin: Alpha determines radius

[Andrea paz]

> on ubuntu 18 (lts?) you can just download and install older libpng

Thanks, I was stuck on atp not finding libpng12....
Now Cin-HV works and I can see that it has the Blur plugin with the
"Alpha determines radius" option. But still I can't understand its
use. Looking at the source code can you understand how it works? It
would be nice to explain it in the manual, or remove it altogether.