How to apply masks to RGB(A) frames that are encoded in their alpha channels?

[Dylon Edwards]

I have an inference pipeline that operates on RGB frames from a RealSense
camera.  The pipeline structure looks something like this:

(realsense) -> (background subtraction) -> (object detection) -> (object
tracking) -> (object classification)

I'm using the segmentation filter for background subtraction, which adds an
alpha channel to the RGB frames and embeds the mask into it (the alpha
channel).  My object detector works on the RGB channels and ignores the
alpha channel, so it ignores the background subtraction.  I need to apply
the alpha channel as a mask to the other channels before passing the frames
into the inference segment of the pipeline.  I could write a custom filter
but would prefer to use one of the existing plugins, are there any existing
plugins that handle this?  I can drop the alpha channel with videoconvert,
but that does me no good since the mask is embedded in the alpha channel.
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