Technically, media-libs/gst-plugins-bad didn't have 'libvisual' useflag, yet media-plugins/gst-plugins-gl did, so after the merge in 1.4 it should have inherited this useflag. This one is for opengl plugin, so distinct from media-plugins/gst-plugins-libvisual and so it should REQUIRE_USE one of opengl flags. PS: while you've eventually added the gtk-doc workaround to the main tarballs, as I noted at the end of the last bug, media-plugins/gst-plugins-libav needs one for its tarball too - while those are bit sketchy (due to the nature of ffmpeg), they're still better than none.
This is not ported. Hence there is nothing to be done here downstream.
Also open a separate report for gtk-doc bugs, we already explained it. Mixing issues does not help in solving them.