ext/pango/gsttextoverlay.*: Add property to configure waiting for text on the textpad or not, with the default behavi...

Original commit message from CVS:
* ext/pango/gsttextoverlay.c: (gst_text_overlay_class_init),
(gst_text_overlay_init), (gst_text_overlay_set_property),
(gst_text_overlay_get_property), (gst_text_overlay_src_event),
(gst_text_overlay_text_event), (gst_text_overlay_video_event),
(gst_text_overlay_text_chain), (gst_text_overlay_video_chain),
(gst_text_overlay_change_state):
* ext/pango/gsttextoverlay.h:
Add property to configure waiting for text on the textpad or not, with
the default behaviour being the old one (always wait for text before
rendering the video). This default behaviour is usually not the best one
because the text stream can very sparse and could require queueing a lot
of video.
Fix the flushing and EOS handing so that we don't mix up their meaning.