basetextoverlay: Remove redundant overlay composition metas

Ensure the outgoing video frame has at most one overlay composition meta. When multiple textoverlay elements are in the same pipeline, the downstream one reuses the overlay composition meta of the upstream one, but the overlay compositor rightfully handles only one overlay composition meta per frame, so the textoverlay needs to get rid of the meta that is being reused from its upstream counterpart.

Fixes #846 (moved)