[Cin] Blur plugin: Alpha determines radius

[Igor BEGHETTO]

Thanks Phyllis for the demo!
It works like the tests I did on a PNG with alpha channel but I think 
the result is more a bug.
Maybe in CinelerraCV it was deleted for that?

> Works in HV 4.6.1 with similar, but not exact results, so that is 
> where it came from (CinGG was based on the original HV and CV mods 
> were added later).
> Not in CV code that I checked.