[Cin] Blur plugin: Alpha determines radius

[Andrea paz]

My test:
I have tried to use the Blur plugin (with "Alpha determines radius")
with a mask and with Blue Banana. The result is always the same
activating or deactivating this option. I tried with the Alpha plugin
and I found a (useless?) difference: with radius =0 it is the same
between active and inactive option (image 1); with radius > 0, with
active option the blur acts normally on the image but the Alpha plugin
is completely deactivated (image 2). If the option is deactivated, the
Alpha plugin continues to work and the Blur acts normally (image 3).

In the CinCV manual you can't find this parameter: either it's
original to CinHV or it's new to CinGG.
I tried to look at the source code in Blur.C, finding references to
"config.a_key" as opposed to just "config.a"; but I can't figure out
how and when it intervenes.
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