[Cin] Blur plugin: Alpha determines radius

[Andrea paz]

I also think it is a bug. If in Phyllis' example we put a track
underneath with, for example, a colored background, activating the
"Alpha determines radius" option makes that track disappear in the
Compositor leaving a black background. It is as if this option took
the black of the first track (which is 100% transparency) and
transformed it into the color black (0, 0, 0), losing the transparency
data and hiding the underlying colored track. The same is achieved
using a gradient.

Maybe the best thing is to hide the option (without needing to remove
it from the source code) so that it is not available.