gstglmixer: copy/past recent changes from gstvideomixer
gstglmixer: copy/past recent changes from gstvideomixer

ystreet/gst-plugins-gl,ystreet/gst-plugins-gl,mapmapteam/gst-plugins-gl,ystreet/gst-plugins-gl,mapmapteam/gst-plugins-gl,mapmapteam/gst-plugins-gl