Transforming GstMeta for saturation overlays

[Josh Doe]

I'd like to create a pair of video filters, the first basically creates a
mask, indicating which pixels are saturated (or just over/under a
threshold), which are "stuck" (haven't changed in the last N frames),
perhaps adding other criteria in the future. A second element would apply
some visualizations to the stream, such as zebra stripes or flashing
colors. I'm aware of the existing zebrastripe element, but it doesn't quite
fit my need, as I typically perform transformations to the video, such as
contrast stretching, mapping 16-bit to 8-bit, or scaling, but want the
criteria applied to the raw input video.

If I create a new GstMeta in the first element, which will contain the
mask, the meta will be invalid if videoscale, videoflip, videocrop, or any
number of other transformations occur.

I see there is a GstMetaTransformFunction, which gave me hope, but I can't
find any usages. Anything I'm missing? Thanks!

-Josh
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