When the image is opaque but the output ProRes format has an alpha component (4 component, 32 bits per pixel), Apple requires that we signal that it should be ignored by setting the depth to 24 bits per pixel. Not doing so causes the encoded files to fail validation.
So we set that in the caps and qtmux sets the depth value in the container, which will be read by demuxers so that decoders can skip those bytes entirely. qtdemux does this, but vtdec does not use this information at present.
The sister change was made in qtmux and qtdemux in: gst-plugins-good!1061 (merged)