basetextoverlay: GL pipeline with multiple overlays renders only the first one

gst-launch-1.0 videotestsrc ! textoverlay text="text1" valignment="top" font-desc="sans bold 30" \
                            ! textoverlay text="text2" halignment="right" font-desc="sans bold 30" \
                            ! textoverlay text="text3" halignment="left" font-desc="sans bold 30" ! glupload ! gtkglsink

Current result:

Expected result:

This is happening because the overlay compositor handles one overlay composition meta per frame, but textoverlay adds a new meta per upstream meta found, so in this pipeline the final video frame has 3 composition metas, each new one storing duplicate infos of the previous.

I have a patch removing redundant metas, will send an MR...